The single largest cost driver is rarely the technology stack — it is uncertainty. Every open question in the brief is converted into a buffer in the estimate. A vendor that has no visibility into the exceptions and edge cases has to assume the more expensive option. Investing a few days in a discovery phase frequently cuts the overall figure much more than haggling over hourly rates.
Integrations tend to be the next major multiplier. A screen that writes to your own database is easy to estimate; the same screen connected to a legacy ERP is a different problem. The effort lives in the third party: rate limits and sandbox access, waiting on someone else’s team, data that does not match your model. Ask each bidder to break integrations out as separate items, since this is where estimates break.
Quality attributes can easily double the estimate. An application used by twenty people is a very different build from the same functionality serving a hundred thousand users. Audit and compliance requirements, .net vs laravel high availability, load handling, node.js development agency data retention rules and localisation add weeks of work. State them early or else expect them to arrive later as change requests.
The team you are quoted matters a great deal. A rate card says very little on its own: a senior engineer at twice the price can be less expensive in the end than two juniors who need heavy code review. Check too which roles are billed: project management, quality assurance, release engineering and analysis are legitimate costs, but these should be visible in the estimate.
The build price is not the total cost. Budget for cloud costs, paid APIs, observability and a change budget annually. A common working assumption is that any production system consumes a meaningful share of the initial investment per year simply to stay current. Ignoring this is the classic mistake.