I agree that estimates are weak, but I can think of two possibilities that might also work: one is the devs set a price, and the other that the users set a bounty. Each has pluses and minuses. The big plus is "all within the same environment, not a separate site".
Software production is craftsmanship
What's needed to achieve targets is not measurable.
And it's not a given that users know what they want, or that they share the vision with the devs
I wouldn't overcomplicate because then implementation gets more difficult and you run into overlaps with uman issues (differences in vision, motivation)
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