The dominant factor is never technology — it remains unclear scope. Every open question in the brief becomes padding inside the number you receive. A team that has no visibility into what happens on the unhappy path must assume the worst. Putting two weeks into a discovery phase frequently cuts the overall figure far more than negotiating the rate.

Integrations tend to be the second big multiplier. A form that saves data is low risk; the same functionality wired into a payment provider and go development outsourcing a CRM is not. The cost sits in the third party: rate limits and sandbox access, long certification processes, data that does not match your model. Ask each bidder to list every external system, because this is the usual source of overruns.

The requirements nobody writes down can easily double the estimate. An application used by twenty people has almost nothing in common with the same functionality handling a hundred thousand users. Security reviews, high availability, load handling, traceability and accessibility add measurable effort. Put them in the brief or expect them priced as extras.

The mix of people behind the number matters. A day rate reveals almost nothing on its own: an experienced hire senior node.js engineer at a premium rate frequently turns out to be cheaper per delivered feature than two inexperienced developers who need constant review. Ask as well what else appears on the invoice: software project cost estimate management, QA, infrastructure work and UX design are real work, but these should be visible in the estimate.

The quoted figure is rarely what you will actually spend. Budget for infrastructure, subscriptions and software development partner licences, observability and a maintenance allowance annually. A useful planning figure says that any production system needs a meaningful share of the original budget every year simply to stay current. Ignoring this is the classic mistake.