A few lines is enough. We'll come back with honest feedback, a rough scope, and what it would take to build it. No obligation.
We read your note properly and reply with what we actually think about the idea, including the parts we think are risky.
How long the work takes and what it depends on, rather than a number designed to win the job.
A clear proposal for the first piece of work, so you can decide whether to go ahead. No obligation either way.