The "Build vs. Buy" Fallacy: The Hidden Maintenance Tax of
Homegrown Testing Stacks
"We have great engineers. Why pay $50k for a tool when we can just build a feature flag system?" Here is the math on why that decision usually fails.
It starts innocently enough. An engineering lead looks at the price tag of a commercial experimentation platform—often $30k to $100k annually—and balks.
"It's just a random number generator and a database," they say. "We can build an MVP in two sprints."
They are technically correct. You can build the MVP in two sprints. But you aren't building a software feature; you are building a statistical engine. And that is where the hidden tax begins.
The "Regret Point" in TCO Analysis
The initial build cost is deceptive. The real cost comes from "Statistical Debugging"—when a PM asks why the data doesn't match Google Analytics, and your best engineer spends three days investigating Sample Ratio Mismatch (SRM).
The Opportunity Cost Calculation
If your Senior Engineer (salary $180k) spends just 20% of their time maintaining this internal tool, that's $36,000/year in direct cost. Add in the opportunity cost of features they didn't build for your core product, and the internal tool is likely costing you $100k+ annually.
The Three Layers of "Build" Complexity
Most teams only budget for Layer 1. Layers 2 and 3 are what kill the project.
- 1
The Assignment Layer (Easy)
Hashing user IDs, assigning buckets, serving variants. This is the "two sprints" part.
- 2
The Statistical Layer (Hard)
Calculating significance, handling outliers, checking for SRM, managing variance reduction (CUPED). One math error here invalidates all your decisions.
- 3
The Governance Layer (Impossible)
Preventing interaction effects between tests, managing feature flag retirement, audit logs. This is a full-time product in itself.
Unless you are Netflix, Airbnb, or Uber, your core competency is not building experimentation infrastructure. For a detailed breakdown of commercial options that scale, see our procurement guide.
The Consultant's Verdict
Buy the tool. The "savings" from building are an illusion paid for by your engineering team's distraction.
The only exception is if your product has unique constraints (e.g., air-gapped environments, extreme latency requirements) that no vendor can meet. Otherwise, let the vendors handle the statistics so you can handle the growth.