The biggest cost driver is not technology — it is unclear scope. Each unanswered question in the brief becomes padding inside the number you receive. A vendor that has no visibility into the edge cases will assume the worst. Putting two weeks into a discovery phase often reduces the total far more than negotiating the rate.
Third-party integrations remain another reliable source of cost. A feature that touches only your own data is easy to estimate; the same functionality connected to an old accounting system is a different problem. The cost hides in the third party: rate limits and sandbox access, waiting on someone else’s team, inconsistent data. Ask any vendor to price integrations separately, since this is the usual source of overruns.
Quality attributes quietly rewrite the estimate. A tool used by a small internal team is a very different build from the same feature set serving public traffic. Audit and compliance requirements, availability guarantees, scalability, llm application development traceability and multi-language support each add measurable effort. Write them down at the start or you can expect the estimate to move later.
The mix of people behind the number changes the arithmetic. An hourly rate tells you little on its own: a senior engineer at a higher rate is often cheaper per delivered feature better than php two juniors who need heavy code review. Also ask what else appears on the invoice: project management, quality assurance, release engineering and analysis are real work, but these should be visible in the estimate.
The quoted figure is rarely what you will actually spend. Expect hosting, subscriptions and licences, logging and alerting and an ongoing support budget for every year the moscow software development agency runs. A common working assumption says that a live system requires a noticeable fraction of the original budget annually simply to stay current. Ignoring this is the most common budgeting mistake.