The biggest cost driver is never the choice of framework — it is unclear scope. Each unanswered question in the requirements is converted into padding in the estimate. A vendor that does not know the edge cases must assume the worst. Putting two weeks into requirements work frequently cuts the total by far more than negotiating the rate.
Connections to other systems tend to be the second big multiplier. A screen that writes to your own database is easy to estimate; the same functionality wired into a payment provider and ai in custom software development a CRM is a different problem. The effort lives in the third party: undocumented APIs, custom python development slow approval cycles, data that does not match your model. Ask any vendor swift web framework to list every external system, since this is where estimates break.
The requirements nobody writes down quietly rewrite the estimate. An application used by a handful of staff costs far less than the same feature set handling thousands of external customers. Compliance work, uptime targets, load handling, data retention rules and localisation add real engineering time. State them early or else expect the estimate to move later.
The mix of people behind the number changes the arithmetic. A day rate reveals little on its own: one senior developer at twice the price is often less expensive in the end than two inexperienced developers who need constant review. Check too which roles are billed: project management, testing, ecommerce web development agency release engineering and design have to be done by someone, but these should be named rather than hidden inside a blended rate.
The number in the proposal is rarely the full cost of ownership. Expect hosting, third-party licences, logging and alerting and an ongoing support budget annually. A useful planning figure is that any production system consumes a noticeable fraction of the initial investment annually in fixes, updates and small changes. Treating the launch as the finish line remains the classic mistake.