The biggest cost driver is not technology — it is almost always how much is still undecided. Each unanswered question in the brief turns into padding in the estimate. A vendor that does not know the exceptions and edge cases must assume the worst. Spending a week on requirements work can cut the total far more than haggling over hourly rates.
Third-party integrations are the second big multiplier. A feature that touches only your own data is low risk; the same functionality talking to an old accounting system is not. The effort sits in the other system: symfony companies rate limits and sandbox access, waiting on someone else’s team, inconsistent data. Ask any vendor to price integrations separately, as that is where the numbers slip.
Non-functional requirements can easily double the number. A tool used by a small internal team costs far less than the same functionality handling thousands of external customers. Audit and compliance requirements, high availability, scalability, traceability and multi-language support all add weeks of work. Put them in the brief or else expect them priced as extras.
Who actually does the work changes the arithmetic. A day rate says almost nothing on its own: an experienced engineer at a higher rate frequently turns out to be cheaper overall than two juniors who require constant review. Check too what else appears on the invoice: coordination, QA, reactjs developer for hire DevOps and UX design are legitimate costs, but they should be visible in the estimate.
The number in the proposal is rarely the full cost of ownership. Plan for infrastructure, native app development paid APIs, observability and an ongoing support budget each year. A reasonable rule of thumb says that a live system consumes a recurring percentage of its original build cost annually simply to stay current. Leaving it out of the budget is the classic mistake.