The single largest cost driver is rarely technology — it is unclear scope. Every ambiguity in the requirements becomes padding inside the number you receive. A supplier that cannot see what happens on the unhappy path must assume the worst. Putting two weeks into a proper discovery frequently cuts the total much more than haggling over hourly rates.
Connections to other systems are another reliable source of cost. A feature that touches only your own data is predictable; the same screen talking to a payment provider and a CRM is another matter entirely. The cost sits in the other system: undocumented APIs, long certification processes, data that does not match your model. Ask any vendor to break integrations out as separate items, since that is where the numbers slip.
Quality attributes can easily double the budget. An internal tool used by a handful of staff has almost nothing in common with the same idea serving public traffic. Security reviews, uptime targets, performance under load, traceability and accessibility add weeks of work. Write them down at the start or else expect them priced as extras.
The mix of people behind the number matters. A day rate reveals almost nothing on its own: an experienced engineer at a higher rate is often less expensive outsourcing versus in house software development the end than two inexperienced hire remote laravel developers who need supervision and rework. Also ask which roles are billed: project management, testing, DevOps and design have to be done by someone, but they should be itemised.
The build price is not the full cost of ownership. Plan for cloud costs, third-party licences, observability and laravel vs django a change budget each year. A reasonable rule of thumb is that a live system needs a meaningful share of the initial investment every year for updates, security patches time and materials vs fixed price contract small improvements. Leaving it out of the budget is the most common budgeting mistake.