Key Takeaways

• The recurring carry settles this, not the build. At $14,165 per fully loaded engineer-month, Simplismart's measured 20 to 30% of a developer's day plus six pricing changes a year runs $73,000 to $90,000 annually.

• Segwise put three weeks of engineering into credit-based pricing in house without shipping it, then went live on Flexprice in three days.

• Building is correct when billing is the product, when a pricing primitive has no vendor implementation, or when a regulator forbids what hosted vendors require.

• Buying costs roadmap control, data location, and a pricing-model ceiling. Source access removes two of the three.

What does building billing in-house actually cost?

It costs an initial build measured in engineer-months plus a recurring carry that never stops, and the carry decides it.

Every figure traces to one input you can swap. The Bureau of Labor Statistics puts the median wage for software developers at $135,980 as of May 2025, or $11,332 a month, and I add 1.25x for tax, benefits, and equipment to reach $14,165 per engineer-month.

The initial-build row is an estimate scaled from the Segwise measurement, so re-scope it. Business cases omit the two recurring rows. We've broken the same arithmetic down at length in the cost of building billing in-house.

When does building billing in-house make sense?

Building wins when the billing logic is the differentiated part of the product, or when no vendor can run where your data has to live.

The cases where I'd still build:

• Billing is the product. Sell billing, payments, or metering to others and the engine is your differentiation.

• A pricing primitive nothing models. Not "our pricing is complicated", which every vendor hears weekly. Closer to a price derived from a proprietary quality score computed inside your own inference pipeline.

• A regulatory position no vendor satisfies. Usage data barred from leaving hardware you control, with nobody shipping a deployable artifact. Self-hosted billing narrows this case, so check it first.

• Pre-revenue on one flat plan. A Stripe Checkout link and a webhook handler is correct there.

• You already built it and prices are frozen. A billing system migration then costs more than the carry, until pricing moves.

What do you give up by buying?

You give up roadmap control, physical control of usage data, a ceiling on expressible pricing models, and exposure to an acquirer.

• Roadmap dependency. The capability you need next quarter arrives when the vendor ships it. You influence the order, you don't set it.

• Data location. Usage events and revenue records sit in the vendor's cloud, a compliance question before a preference. Open source billing reverses that, because a self-hostable vendor keeps the data in your infrastructure.

• A pricing-model ceiling. Platforms model some pricing shapes natively and others by workaround. A hybrid pricing model putting credits, committed usage, and metered overage on one invoice is where ceilings surface.

• Acquisition risk. Adyen acquired Orb in July 2026 and Stripe acquired Metronome in January 2026. Closed source means the roadmap changes owners without you.

Flexprice is enterprise-grade, open source usage based billing infrastructure for AI and SaaS companies. It can be deployed in your own VPC, on-prem, or on Flexprice's managed cloud. Segwise budgeted three weeks for credit-based pricing in house, never shipped it, went live on Flexprice in three days, and now runs credit infrastructure for 100+ enterprise customers with no engineer assigned. Seat-based, usage-based, credit-based, and hybrid structures are configuration in Pricing Models.

"Our core product is not credits. We build ad analysis and generation technology, not billing infrastructure, and that is where my focus needs to be." - Kush Daga, Founding Engineer, Segwise

Try it free at us.flexprice.io/auth?tab=signup.

What questions settle the decision?

These questions settle it, and the subscription price isn't one.

• How many pricing changes ship in the next twelve months? Zero or one favours building. Near-monthly puts engineering in front of every revenue experiment.

• Can your usage data legally sit in a vendor's cloud? If not, the question becomes buy and self-host.

• What breaks if one month of invoices is wrong? An audit clause makes correctness a compliance exposure, raising the bar on the billing engine past the happy path most builds scope.

• Who carries the pager for billing at 2am? With no name there, the recurring cost is unfunded.

Related terms

Terms that sit either side of this decision:

• Billing Engine names the thing you're deciding whether to write yourself.

• Billing System Migration is the work that follows a decision to buy.

• Open Source Billing is the middle path between the two options.

• Self-Hosted Billing answers the data-residency version of this question.

• Hybrid Pricing Model is the pricing shape that most often exceeds a platform's ceiling.

• Feature Gating is the entitlement work teams forget to scope into a build.

FAQ

Is it cheaper to build or buy a billing system?

Buying is cheaper for almost every team once the carry counts. A build front-loads roughly $113,320, then adds $73,000 to $90,000 a year a platform fee replaces.

How long does it take to build a billing system?

Longer than the invoice-generation path teams scope, because corrections and edge cases are the bulk of it. Segwise gave credits alone three weeks and didn't ship.

Can you start with Stripe and build billing later?

For a pre-revenue product on one flat plan, that's the right order. The decision point arrives when pricing stops being flat: metered overage, prepaid credits, negotiated terms.