The biggest cost driver is rarely technology — it is how much is still undecided. Every open question in the requirements is converted into a buffer inside the number you receive. A vendor symfony web developer that has no visibility into the exceptions and edge cases will assume a pessimistic case. Spending a week on a proper discovery frequently cuts the total far more than negotiating the rate.
Third-party integrations tend to be another reliable source of cost. A screen that writes to your own database is easy to estimate; the same functionality talking to a legacy ERP is not. The effort lives in the third party: rate limits and sandbox access, long certification processes, data that does not match your model. Ask the estimator to list every external system, since this is the usual source of overruns.
Non-functional requirements quietly rewrite the number. An internal tool used by a small internal team costs far less than the same feature set serving public traffic. Security reviews, availability guarantees, load handling, audit logging and localisation add measurable effort. State them early or else expect the estimate to move later.
The mix of people behind the number matters. A rate card reveals little on its own: a senior engineer at a premium rate is often cheaper per delivered feature than two inexperienced hire developers in germany who need constant review. Check too which roles are billed: project management, QA, web development company moscow infrastructure work and design have to be done by someone, but they should be visible in the estimate.
The build price is not the full cost of ownership. Expect infrastructure, paid APIs, monitoring and a change budget each year. A common working assumption holds that any production system needs a noticeable fraction of the initial investment per year for web application development company updates, security patches and small improvements. Ignoring this has always been the classic mistake.