Non-functional requirements silently change the number. An application used by twenty people is a very different build from the same idea handling a hundred thousand users.
Salvadorbrak
Define what the word done means feature by feature. Clear acceptance criteria need not use special syntax: a short list stating what a user should be able to do is enough.
Handing a project to a vendor implies an external team owns the outcome: the provider staffs the roles, the partner manages the plan, and they absorb the staffing risk.
A few questions usually settle it. First: is what you are building the product itself, or a supporting tool? Then: over what horizon will you need this capacity — months or years? Third: who will maintain it in two years?
One last thing, say what you expect back. Ask for a task-level breakdown, the assumptions behind each number, whatever the team considers risky and a low number and a high number.
An estimate that arrives instantly is a bad sign. A competent team returns clarifying questions before any number: about integrations.
Third-party integrations are the second big multiplier. A form that saves data is predictable; the same screen talking to a payment provider and a CRM is a different problem.