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Exactly, since value is subjective, people should be able to assign a max value to something =)
(At least regarding what they spent on it)

In your example, what happens if there is a fourth upvoter with a max value of $60?

It would work like a 0% upvote.
He'd upvote but it wouldn't change the value.
If he'd upvote with 90$, the one with 90$ would receive a part of his back.

But, as another user mentioned, this system still needs to be adapted, to, for example allow space for curation.

I think a solution would be calculating a "mean-value" of the existing, and allowing to go up to this to avoid 0% votes, to guarantee curation.