The dominant factor is not the choice of framework — it is almost always unclear scope. Each unanswered question in the brief turns into a buffer in the estimate. A supplier that cannot see the exceptions and edge cases will assume the worst. Spending a week on a discovery phase often reduces the overall figure far more than haggling over hourly rates.
Integrations are another reliable source of cost. A form that saves data is low risk; the same feature wired into a legacy ERP is a different problem. The unknown lives in the counterparty: poor react app development company documentation, waiting on someone else’s team, fields that mean something different on each side. Ask any vendor to break integrations out as separate items, since that is where the numbers slip.
The requirements nobody writes down silently change the budget. An internal tool used by twenty people costs far less than the same functionality handling public traffic. Compliance work, uptime targets, scalability, data retention rules and multi-language support add weeks of work. Put them in the brief or expect them priced as extras.
The team you are quoted matters a great deal. A day rate tells you little on its own: an experienced engineer at a higher rate can be cheaper per delivered feature than two inexperienced developers who need heavy code review. Also ask who else is billed: delivery management, quality assurance, release engineering and UX design have to be done by someone, but they must be itemised.
The build price is rarely the total cost. Plan for hosting, paid APIs, monitoring and a change budget for hire smm specialists every year the software runs. A common working assumption is that any production system needs a noticeable fraction of its original build cost annually in fixes, updates and small changes. Leaving it out of the budget has always been the most frequent planning error.