The single largest cost driver is not technology — it is uncertainty. Each unanswered question in the requirements becomes a buffer somewhere in the quote. A supplier that has no visibility into the edge cases will assume the more expensive option. Putting two weeks into a discovery phase frequently cuts the overall figure much more than haggling over hourly rates.
Integrations remain another reliable source of cost. A form that saves data is low risk; the same functionality talking to a legacy ERP is another matter entirely. The effort hides in the third party: symfony web developer undocumented APIs, slow approval cycles, inconsistent data. Ask each bidder to price integrations separately, because this is the usual source of overruns.
Non-functional requirements silently change the budget. An application used by twenty people costs far less than the same idea serving thousands of external customers. Security reviews, high availability, load handling, data retention rules and localisation all add weeks of work. Put them in the brief or else expect them priced as extras.
The team you are quoted matters a great deal. A rate card reveals very little on its own: an experienced engineer at twice the price frequently turns out to be cheaper overall than two juniors who need heavy code review. Check too who else is billed: coordination, QA, infrastructure work and livewire developer UX design are real work, but they must be named rather than hidden inside a blended rate.
The number in the proposal is not the total cost. Plan for hosting, subscriptions and licences, observability and a maintenance allowance for every year the crypto futures trading software development runs. A useful planning figure is that any production system requires a noticeable fraction of its original build cost per year simply to stay current. Ignoring this is the classic mistake.