The biggest cost driver is not the technology stack — it is almost always how much is still undecided. Each unanswered question in the brief is converted into padding in the estimate. A supplier that cannot see the edge cases has to assume the worst. Investing a few days in requirements work can cut the total far more than any rate negotiation.
Third-party integrations tend to be the next major multiplier. A feature that touches only your own data is low risk; the same functionality connected to a legacy ERP is not. The effort hides in the other system: poor why mvps fail documentation, waiting on someone else’s team, data that does not match your model. Ask the estimator to list every external system, as that is where the numbers slip.
Quality attributes silently change the number. An application used by a small internal team is a very different build from the same feature set handling public traffic. Compliance work, availability guarantees, load handling, audit logging and multi-language support add weeks of work. Write them down at the start or else expect the estimate to move later.
Who actually does the work matters a great deal. An hourly rate tells you almost nothing on its own: one senior custom development insights developer at twice the price is often less expensive in the end than two juniors who require constant review. Check too which roles are billed: project management, QA, release engineering and UX design are real work, but these should be named rather than hidden inside a blended rate.
The quoted figure is never what you will actually spend. Budget for hosting, third-party licences, monitoring and a maintenance allowance annually. A common working assumption holds that any production system consumes a meaningful share of the original budget annually simply to stay current. Ignoring this has always been the most common budgeting mistake.