Most founders underestimate what login actually is. It looks like a form with two fields. Under the hood it is one of the more complex pieces of a product, and one of the most consequential. A bug in your checkout flow costs you a sale. A bug in your auth system can cost you every user account you have ever created.
The good news: this is well-understood engineering territory. Costs are predictable, timelines are tight, and the decisions are clearer than they appear.
What goes into a full authentication and account system?
Login is not one feature. It is a collection of related features that each take real time to build and test.
Email-and-password login covers sign-up, sign-in, and password reset at minimum. Sign-up alone involves email validation, duplicate-account detection, and a confirmation email flow. Password reset means a secure one-use link that expires after a set window. If you get the expiry logic wrong, you have a security hole. None of this is complex, but each piece needs to be built and tested carefully.
On top of that, most products need separate permission levels. An admin user who can see everything. A regular user who can only see their own data. A guest or trial user with read-only access. Engineering these roles correctly, so that a regular user cannot accidentally access another user's records, is one of the more subtle problems in the whole system. According to the 2021 Verizon Data Breach Investigations Report, broken access control was the most common application vulnerability, present in 94% of tested applications. Getting permission logic right is not optional.
Account management rounds out the feature set: editing a profile, changing an email address, uploading an avatar, deleting an account. These look minor but each touches the same data model that login depends on. A well-built account system takes roughly 80–120 hours of senior engineering time. At a Western agency billing $150–$200/hour, that is $12,000–$24,000 before any overhead.
A global engineering team at Timespade delivers the same scope for $3,000–$6,000, with the same test coverage and the same production-grade quality.
How does OAuth-based social login work under the hood?
Social login ("Sign in with Google," "Sign in with Apple") is the single most-requested auth feature after basic email login. Founders want it because it removes friction: no password to forget, no email to confirm, one click and the user is in.
What actually happens when a user clicks that button is a handoff. Your app sends the user to Google. Google confirms the user's identity and sends a cryptographically signed token back to your app. Your app verifies the token, extracts the user's email and name, and either creates a new account or logs them into an existing one.
The underlying standard that governs all of this is called OAuth 2.0, combined with an identity layer called OpenID Connect. You do not need to understand the protocol. What matters is what can go wrong if it is implemented carelessly.
Two failure modes come up repeatedly in audits. First, account-merging logic: a user signs in with Google using the same email they previously registered with a password. If your app creates a duplicate account instead of linking them, the user suddenly has two separate profiles with no shared data. Second, token validation: if your server accepts a token without checking its authenticity, an attacker can forge one. Both issues are avoidable with a thorough implementation, but they require deliberate handling.
Adding Google and Apple social login to an existing auth system typically adds $1,500–$3,000 to the build cost. Each provider involves reading their documentation, handling edge cases specific to their flow, and testing across browsers and mobile devices where behavior sometimes differs.
Should I build auth from scratch or use a managed service?
This is the most consequential decision in the whole auth conversation, and the answer depends less on budget than founders expect.
Managed auth services (Auth0, Firebase Authentication, Supabase Auth) handle the cryptography, token storage, and security maintenance for you. You integrate their library and your app gets login, social sign-in, and basic account management in a day or two. The service monitors for breaches and patches vulnerabilities automatically. For many early-stage products, this is the right call.
Auth0's free tier supports 7,000 monthly active users. Firebase Authentication is free for email and social login at any volume. These numbers cover most MVPs through their first year without a dollar of spend.
The tradeoff is control and long-term cost. Auth0's Growth plan starts at $23/month and scales to $240+/month at 1,000 monthly active users. At 10,000 users the bill is typically $1,000–$1,500/month. That is a recurring cost that compounds. Custom-built auth costs more upfront ($5,000–$8,000 for a complete system) but costs nothing to run after that, and you own every part of it.
The rule of thumb: use a managed service for your MVP and your first 6–12 months of growth. Build custom auth when your monthly active users cross 5,000–10,000 and the managed service bill starts looking meaningful relative to your revenue. At Timespade, we migrate clients from managed auth to custom auth regularly; the migration itself typically costs $4,000–$6,000 and pays for itself within 12–18 months at scale.
| Approach | Setup cost | Monthly cost at 10K users | Control | Maintenance |
|---|---|---|---|---|
| Managed service (Auth0, Firebase) | $1,500–$3,000 | $500–$1,500/mo | Low | Handled by vendor |
| Custom-built auth | $5,000–$10,000 | $0–$50/mo (infrastructure only) | Full | Your team or agency |
| Hybrid (managed now, custom later) | $1,500–$3,000 now + $4,000–$6,000 migration | $0–$50/mo after migration | Full after migration | Low |
For most pre-revenue founders, the managed service route is cheaper at the start and faster to ship. The migration path exists and is well-defined. There is no reason to over-engineer auth before you have paying users.
What does adding two-factor authentication cost?
Two-factor authentication (2FA) asks a user to prove their identity with a second piece of evidence after entering their password, usually a six-digit code sent by text message or generated by an app on their phone.
Founding teams often defer 2FA until a security incident forces the conversation. That is a mistake for two reasons. First, credential stuffing attacks (where attackers try username/password pairs leaked from other breached products) are automated and continuous. Microsoft's 2021 security report found that 2FA blocks 99.9% of automated account attacks. Second, certain business customers and enterprise buyers will not sign a contract without it.
Building SMS-based 2FA requires integrating a messaging provider (Twilio is the most common), setting up the code generation and expiry logic, and handling edge cases like lost phones and account recovery. That adds roughly $1,500–$2,500 to a project.
App-based 2FA (Google Authenticator, Authy) is actually cheaper to build than SMS. It uses a standard algorithm that does not require a third-party messaging service, so there is no per-message cost and no dependency on SMS delivery rates. Implementation adds $800–$1,500 to the project cost.
The ongoing operational cost difference matters too. SMS 2FA costs roughly $0.0075–$0.01 per message sent. At 10,000 logins per month with 2FA enabled, that is $75–$100/month. App-based 2FA has no per-use cost after setup.
| 2FA method | Build cost | Per-use cost | User experience |
|---|---|---|---|
| SMS one-time code | $1,500–$2,500 | $0.0075–$0.01/message | Familiar; works without a smartphone |
| Authenticator app (TOTP) | $800–$1,500 | $0 after setup | Slightly more setup friction; no recurring cost |
| Both options (user's choice) | $2,000–$3,500 | Mixed | Best coverage; recommended for B2B products |
For consumer products, SMS 2FA is the safer default. Most users already know the flow. For B2B products where users are more technical, offering both options is standard and worth the marginal extra cost.
How much ongoing maintenance does an auth system need?
Auth is not a feature you build and forget. Dependencies get patched, token libraries release security fixes, and the browsers and mobile operating systems your users rely on change their behavior over time.
A realistic maintenance estimate for a custom auth system is 4–8 hours per month. That covers reviewing security advisories for the libraries your auth code depends on, applying patches, and occasionally adjusting behavior when a browser update changes how login sessions work. At a senior engineer's rate of $50–$80/hour, that is $200–$640/month, or $2,400–$7,700/year.
With a managed service, the vendor handles all of that. You trade the monthly bill for freedom from maintenance overhead. For a small team with no dedicated security engineering, that trade is often worth it even at scale.
One category of maintenance that surprises founders is account recovery. Users lose phones, forget they used social login instead of email, and occasionally get locked out through no fault of the product. Building a proper account recovery flow (one that verifies identity before unlocking an account without creating a vulnerability) typically adds $1,000–$2,000 to the initial build.
For a product with 10,000 active users, budget roughly $500–$800/month for auth maintenance if you run custom infrastructure, or $800–$1,500/month for a managed service that eliminates the maintenance burden. The total cost of ownership over two years lands in a similar range either way.
A well-scoped auth system built at the start of a project (with proper permission logic, social login, and at least one 2FA option) typically costs $8,000–$12,000 with a global engineering team. Western agencies quote $25,000–$40,000 for the same scope. The engineering work is identical; the overhead is not. If you want to scope your auth system before writing a single line of code, book a free discovery call with Timespade here.
