The single largest cost driver is not technology — it is how much is still undecided. Each unanswered question in the specification is converted into a contingency in the estimate. A vendor that does not know the edge cases must assume the worst. Putting two weeks into a discovery phase can cut the total by far more than haggling over hourly rates.
Integrations tend to be the next major multiplier. A form that saves data is easy to estimate; the same feature connected to a legacy ERP is not. The unknown sits in the other system: undocumented APIs, waiting on someone else’s team, inconsistent data. Ask each bidder to list every external system, as that is where the numbers slip.
The requirements nobody writes down can easily double the estimate. A tool used by a small internal team has almost nothing in common with the same idea serving a hundred thousand users. Compliance work, uptime targets, load handling, traceability and localisation add real engineering time. Write them down at the start or expect them to arrive later as change requests.
The mix of people behind the number matters. A rate card tells you very little on its own: one senior developer at a premium rate frequently turns out to be cheaper overall than two juniors who need supervision and rework. Also ask who else is billed: hire rabbitmq programmer coordination, QA, infrastructure work and design have to be done by someone, but they must be named rather than hidden inside a blended rate.
The quoted figure is never the full cost of ownership. Plan for infrastructure, turnkey software development services subscriptions and licences, kubernetes web development company monitoring and an ongoing support budget for every year the software runs. A common working assumption says that software in active use requires a meaningful share of its original build cost every year in fixes, updates and small changes. Treating the launch as the finish line has always been the most frequent planning error.