The single largest cost driver is rarely the technology stack — it is how much is still undecided. Every ambiguity in the requirements turns into a contingency somewhere in the quote. A vendor that does not know the edge cases must assume the more expensive option. Putting two weeks into a discovery phase can cut the overall figure by far more than negotiating the rate.
Connections to other systems are the second big multiplier. A screen that writes to your own database is easy to estimate; the same functionality talking to an old accounting system is not. The unknown sits in the counterparty: hire flutter ui designers undocumented APIs, slow approval cycles, inconsistent data. Ask any vendor to price integrations separately, since this is where estimates break.
The requirements nobody writes down quietly rewrite the number. An internal tool used by twenty people has almost nothing in common with the same idea serving thousands of external customers. Security reviews, high availability, performance under load, audit logging and multi-language support all add real engineering time. Write them down at the start flutter or react native else expect them to arrive later as change requests.
Who actually does the work changes the arithmetic. A rate card reveals little on its own: an experienced engineer at twice the price frequently turns out to be less expensive in the end than two juniors who need supervision and software development outsourcing moscow rework. Ask as well what else appears on the invoice: delivery management, quality assurance, DevOps and analysis are legitimate costs, but they should be visible in the estimate.
The number in the proposal is never the full cost of ownership. Expect cloud costs, subscriptions and licences, logging and alerting and a change budget each year. A common working assumption says that any production system consumes a meaningful share of the initial investment annually simply to stay current. Leaving it out of the budget is the classic mistake.