An estimate that arrives instantly should be treated as a bad sign. Any serious team responds with clarifying questions before any number: about integrations. A provider that quotes without asking anything is pricing a guess, and the gap resurfaces as a change order — on your budget.
Watch for any distance between the team in the pitch and the developers actually assigned. Request the names and CVs of the actual team in the contract, web app development cost with a provision about substitutions. A vendor that will only describe roles and refuses to name individuals is keeping the right to assign anyone it likes.
Ask for access to the repository from day one. A partner that shows code only at milestones expects you to accept a black box. Daily commits show you the actual pace far better than a weekly report. The same holds for the automated test suite: if nothing runs automatically, assurances about quality remain just talk.
Ambiguous wording in the contract around intellectual property is never a formality. The agreement must state plainly that all outputs produced under it belong to the client as they are paid for. Also check the governing law and the milestone terms: a large upfront payment with no milestone tied to it takes away your only leverage.
Lastly, request a development proposal pay attention to the working rhythm. Establish how much working-time overlap you will share with your working day, which named person is expected to answer questions and how quickly. Some genuine overlap is normally sufficient; none at all converts each small question into a day of delay. Unclear written communication in the sales phase will not improve later.