The single largest cost driver is rarely technology consulting company — it is almost always unclear scope. Every open question in the specification becomes a buffer inside the number you receive. A supplier that cannot see what happens on the unhappy path has to assume a pessimistic case. Spending a week on requirements work can cut the final cost much more than negotiating the rate.
Connections to other systems tend to be another reliable source of cost. A screen that writes to your own database is low risk; the same functionality connected to an old accounting system is a different problem. The unknown hides in the other system: poor documentation, long certification processes, inconsistent data. Ask any vendor to price integrations separately, as this is the usual source of overruns.
Quality attributes silently change the number. An internal tool used by a handful of staff costs far less than the same functionality serving thousands of external customers. Compliance work, high availability, performance under load, data retention rules and accessibility each add measurable effort. State them early or you can expect them priced as extras.
Who actually does the work changes the arithmetic. A rate card reveals almost nothing on its own: one senior developer at a higher rate frequently turns out to be cheaper per delivered feature than two juniors who need heavy code review. Also ask what else appears on the invoice: project management, testing, DevOps and analysis have to be done by someone, but these should be visible in the estimate.
The build price is rarely the full cost of ownership. Plan for infrastructure, third-party licences, observability and a change budget for hire activemq developers every year the software runs. A reasonable rule of thumb is that software in active use consumes a recurring percentage of the initial investment per year for updates, security patches and small improvements. Leaving it out of the budget has always been the classic mistake.