The single largest cost driver is not technology — it is almost always unclear scope. Every ambiguity in the brief is converted into a contingency in the estimate. A team that does not know the exceptions and edge cases will assume the worst. Putting two weeks into a discovery phase frequently cuts the overall figure much more than negotiating the rate.
Connections to other systems are the second big multiplier. A screen that writes to your own database is low risk; the same screen talking to an old accounting system is another matter entirely. The cost sits in the other system: undocumented APIs, java development outsourcing long certification processes, fields that mean something different on each side. Ask the estimator to price integrations separately, because this is the usual source of overruns.
Non-functional requirements quietly rewrite the number. An application support and maintenance services used by a small internal team has almost nothing in common with the same idea handling thousands of external customers. Audit and compliance requirements, availability guarantees, scalability, data retention rules and accessibility all add weeks of work. Write them down at the start or expect them to arrive later as change requests.
The mix of people behind the number matters. A day rate tells you very little on its own: an experienced engineer at a premium rate is often cheaper overall than two inexperienced developers who require constant review. Also ask what else appears on the invoice: project management, QA, infrastructure work and UX design are legitimate costs, but these should be visible in the estimate.
The build price is not the total cost. Plan for infrastructure, paid APIs, monitoring and a change budget annually. A reasonable rule of thumb says that software in active use needs a meaningful share of the initial investment per year for updates, security patches and small improvements. Treating the launch as the finish line is the most common budgeting mistake.