The dominant factor is rarely the technology stack — it is almost always unclear scope. Every open question in the requirements turns into a contingency in the estimate. A team that has no visibility into what happens on the unhappy path must assume the more expensive option. Spending a week on requirements work frequently cuts the total by far more than haggling over hourly rates.
Connections to other systems remain the second big multiplier. A feature that touches only your own data is low risk; the same screen connected questions to ask a software development company an old accounting system is another matter entirely. The unknown sits in the other system: rate limits and sandbox access, .net consulting services slow approval cycles, inconsistent data. Ask the estimator to break integrations out as separate items, because that is where the numbers slip.
Quality attributes quietly rewrite the budget. An application used by twenty people is a very different build from the same functionality handling thousands of external customers. Compliance work, availability guarantees, load handling, traceability and multi-language support all add measurable effort. State them early or you can expect them priced as extras.
The mix of people behind the number changes the arithmetic. A rate card tells you very little on its own: an experienced engineer at a premium rate frequently turns out to be cheaper per delivered feature than a pair of junior developers who need heavy code review. Also ask what else appears on the invoice: coordination, quality assurance, infrastructure work and design are legitimate costs, but they should be visible in the estimate.
The build fixed price contract software development is rarely the total cost. Expect cloud costs, third-party licences, monitoring and a change budget for every year the software development company in usa runs. A common working assumption is that any production system consumes a meaningful share of the initial investment every year simply to stay current. Treating the launch as the finish line is the most frequent planning error.