The dominant factor is rarely the technology stack — it remains how much is still undecided. Each unanswered question in the requirements is converted into a contingency somewhere in the quote. A supplier that cannot see what happens on the unhappy path will assume the more expensive option. Investing a few days in a proper discovery can cut the overall figure far more than any rate negotiation.

Third-party integrations are another reliable source of cost. A screen that writes to your own database is predictable; the same screen connected to a legacy ERP is a different problem. The unknown sits in the third party: undocumented APIs, long certification processes, inconsistent data. Ask any vendor to break integrations out as separate items, since this is the usual source of overruns.

The requirements nobody writes down quietly rewrite the number. An application used by twenty people has almost nothing in common with the same idea handling a hundred thousand users. Security reviews, uptime targets, load handling, traceability and accessibility all add weeks of work. Write them down at the start or hire i18next developers else expect them priced as extras.

The mix of people behind the number changes the arithmetic. An hourly rate reveals almost nothing on its own: an experienced engineer at twice the price frequently turns out to be cheaper overall than two juniors who require heavy code review. Check too which roles are billed: project management, QA, ecommerce development company infrastructure work and analysis are legitimate costs, but they should be itemised.

The number in the proposal is rarely what you will actually spend. Plan for cloud costs, subscriptions and licences, logging and alerting and an ongoing support budget for every year the software runs. A reasonable rule of thumb holds that a live system requires a noticeable fraction of the original budget every year for updates, security patches and small improvements. Leaving it out of the budget is the most common budgeting mistake.