The single largest cost driver is never technology — it is how does livewire work much is still undecided. Each unanswered question in the specification turns into a buffer somewhere in house team vs outsourcing costs the quote. A supplier that does not know what happens on the unhappy path will assume the more expensive option. Investing a few days in requirements work frequently cuts the overall figure by far more than any rate negotiation.
Third-party integrations are another reliable source of cost. A screen that writes to your own database is predictable; the same screen wired into a payment provider and a CRM is another matter entirely. The cost sits in the other system: poor documentation, long certification processes, data that does not match your model. Ask the estimator to price integrations separately, because this is where estimates break.
Quality attributes can easily double the number. A tool used by a small internal team has almost nothing in common with the same idea serving public traffic. Security reviews, availability guarantees, load handling, traceability and localisation all add weeks of work. State them early or expect them priced as extras.
The mix of people behind the number matters a great deal. A day rate says almost nothing on its own: an experienced engineer at a premium rate can be cheaper per delivered feature than a pair of junior developers who require supervision and rework. Check too which roles are billed: delivery management, testing, infrastructure work and design are legitimate costs, but these should be named rather than hidden inside a blended rate.
The build price is rarely the full cost of ownership. Budget for hosting, subscriptions and licences, logging and alerting and a maintenance allowance annually. A useful planning figure holds that a live system needs a recurring percentage of the initial investment annually for updates, security patches and small improvements. Ignoring this remains the most frequent planning error.