One idea for you is to have a minimum and a maximum funding target, and allow applicants to describe what they plan to accomplish at each of those extremes.

If the campaign doesn't reach the minimum target, the funds should be returned to the patrons (I think you have to figure out a solution for that, regardless). However, dependent on where the project lands on the scale between minimum and maximum, patrons can have reasonable expectations as to the output of the project.

Also, there's an opportunity for stretch goals at certain milestones beyond the maximum funding target. If you wanted to do that, I'd just scrap the maximum funding target entirely, and just have a minimum with multiple stretch goals beyond it.

Again, I love the problem you're trying to solve, keep at it!