The dominant factor is never the technology stack — it is how much is still undecided. Every ambiguity in the specification turns into a buffer inside the number you receive. A vendor that cannot see the edge cases must assume the more expensive option. Spending a week on a discovery phase can cut the total much more than negotiating the rate.
Connections to other systems are the second big multiplier. A screen that writes to your own database is predictable; the same functionality connected to a payment provider and a CRM is another matter entirely. The cost hides in the other system: poor hire senior node.js developer documentation, blockchain development services long certification processes, inconsistent data. Ask the estimator to price integrations separately, since this is where estimates break.
The requirements nobody writes down can easily double the budget. An internal tool used by twenty people costs far less than the same functionality serving a hundred thousand users. Compliance work, uptime targets, load handling, data retention rules and localisation all add real engineering time. Write them down at the start flutter or react native else expect them to arrive later as change requests.
The mix of people behind the number matters a great deal. A day rate tells you very little on its own: a senior engineer at a higher rate frequently turns out to be cheaper per delivered feature than two juniors who require supervision and rework. Ask as well who else is billed: project management, testing, release engineering and design have to be done by someone, but they should be itemised.
The quoted figure is not the total cost. Plan for cloud costs, paid APIs, observability and a change budget for every year the software outsourcing models runs. A common working assumption is that any production system needs a recurring percentage of the original budget per year for updates, security patches and small improvements. Leaving it out of the budget has always been the most frequent planning error.