What will it actually cost to build?

It gets you to the right order of magnitude, which is what you need for a budget conversation. It cannot know your integrations, your compliance regime or the state of the system you are extending — and those three are what move a number by fifty percent. Treat it as a starting range, not a quote.

Number of distinct user roles, integration count, whether anything is regulated, how many platforms you ship to, and how much of the design already exists. Feature count matters far less than people expect; roles and integrations matter far more.

Usually because they are quoting different things. One includes design, testing, deployment and three months of support; another quotes build only and charges for the rest later. Ask every vendor what is excluded — that question is more revealing than the number.

Yes, against a written scope, and that is how most of our first engagements run. Fixed price needs a fixed definition of done, so it follows discovery rather than preceding it. Quoting a fixed price without discovery means padding it, which you pay for.