The single largest cost driver is never technology — it remains uncertainty. Every ambiguity in the specification turns into padding inside the number you receive. A team 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 by far more than haggling over hourly rates.
Connections to other systems are another reliable source of cost. A screen that writes to your own database is predictable; the same feature talking to a legacy ERP is another matter entirely. The cost hides in the counterparty: undocumented APIs, slow approval cycles, inconsistent data. Ask any vendor to break integrations out as separate items, because that is where the numbers slip.
The requirements nobody writes down quietly rewrite the number. An internal tool used by a handful of staff is a very different build from the same idea serving thousands of external customers. Audit and compliance requirements, high availability, load handling, traceability and accessibility all add real engineering time. Put them in the brief or else expect them to arrive later as change requests.
The mix of people behind the number matters a great deal. A day rate says almost nothing on its own: a senior engineer at a premium rate is often less expensive in the end than two inexperienced developers who require supervision and rework. Also ask what else appears on the invoice: coordination, QA, DevOps and design are legitimate costs, but these should be named rather than hidden inside a blended rate.
The number in the proposal is rarely the total cost. Expect cloud costs, paid APIs, logging and alerting and a change budget for every year the software development company for startups runs. A useful planning figure is that software development outsourcing usa in active use needs a recurring percentage of the original budget annually simply to stay current. Ignoring this remains the most common budgeting mistake.