The dominant factor is never the choice of framework — it remains uncertainty. Every ambiguity in the specification becomes a contingency inside the number you receive. A team that has no visibility into the exceptions and edge cases has to assume the worst. Investing a few days in a discovery phase can cut the overall figure much more than any rate negotiation.
Connections to other systems tend to be the second big multiplier. A form that saves data is predictable; the same functionality wired into a legacy ERP is not. The cost hides in the other system: rate limits and sandbox access, waiting on someone else’s team, react development agency fields that mean something different on each side. Ask the estimator technology stack for web apps to break integrations out as separate items, since that is where the numbers slip.
The requirements nobody writes down silently change the estimate. An application used by a small internal team costs far less than the same idea handling a hundred thousand users. Compliance work, uptime targets, load handling, data retention rules and multi-language support all add real engineering time. Write them down at the start saas or custom development you can expect them priced as extras.
The mix of people behind the number changes the arithmetic. A day rate reveals very little on its own: an experienced engineer at twice the price can be less expensive in the end than two juniors who need supervision and rework. Ask as well who else is billed: project management, quality assurance, DevOps and design have to be done by someone, but they should be named rather than hidden inside a blended rate.
The quoted figure is never what you will actually spend. Expect infrastructure, hiring full stack developer subscriptions and licences, monitoring and a change budget annually. A common working assumption is that any production system requires a recurring percentage of the original budget annually in fixes, updates and small changes. Ignoring this has always been the classic mistake.