The biggest cost driver is never the technology stack — it remains how much is still undecided. Every open question in the requirements becomes a buffer in the estimate. A team that has no visibility into the exceptions and edge cases will assume the more expensive option. Investing a few days in a discovery phase can cut the overall figure far more than negotiating the rate.
Third-party integrations tend to be another reliable source of cost. A form that saves data is low risk; the same feature talking to a payment provider software maintenance and support services a CRM is another matter entirely. The unknown lives in the third party: rate limits and sandbox access, long certification processes, data that does not match your model. Ask any vendor to break integrations out as separate items, since this is where estimates break.
The requirements nobody writes down silently change the number. An internal tool used by twenty people costs far less than the same functionality serving public traffic. Audit and compliance requirements, uptime targets, performance under load, data retention rules and localisation add real engineering time. Write them down at the start or else expect them priced as extras.
The mix of people behind the number matters a great deal. An hourly rate tells you little on its own: an experienced engineer at a higher rate can be less expensive in the end than two inexperienced developers who need heavy code review. Check too what else appears on the invoice: project management, QA, enterprise application development in java release engineering and UX design are legitimate costs, but they should be visible in the estimate.
The build price is not the full cost of ownership. Expect hosting, subscriptions and licences, logging and alerting difference between php and python a change budget each year. A useful planning figure holds that custom software development cost in active use needs a meaningful share of the original budget annually for updates, security patches and small improvements. Ignoring this is the classic mistake.