The biggest cost driver is rarely technology — it is almost always how much is still undecided. Every ambiguity in the specification is converted into padding somewhere in the quote. A vendor how software outsourcing works that cannot see what happens on the unhappy path will assume the more expensive option. Spending a week on a discovery phase frequently cuts the total much 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 connected to an old accounting system is a different problem. The cost sits in the third party: poor documentation, slow approval cycles, fields that mean something different on each side. Ask each bidder to price integrations separately, as this is where estimates break.
The requirements nobody writes down can easily double the budget. A tool used by a handful of staff costs far less than the same functionality serving a hundred thousand users. Compliance work, uptime targets, performance under load, audit logging and multi-language support add weeks of work. Put them in the brief or else expect them priced as extras.
Who actually does the work changes the arithmetic. An hourly rate tells you little on its own: laravel vs nextjs a senior engineer at a higher rate is often cheaper per delivered feature than two juniors who need heavy code review. Also ask what else appears on the invoice: coordination, quality assurance, release engineering and design have to be done by someone, but they must be visible in the estimate.
The quoted figure is not what you will actually spend. Expect cloud costs, third-party licences, observability and a change budget each year. A useful planning figure holds that a live system consumes a recurring percentage of the initial investment every year simply to stay current. Leaving it out of the budget remains the classic mistake.