The problem is that we are asking voters to evaluate a post's value, but the only answers available are "more" and "less".

I'm not sure about the scalability or implementation complexity, but I think you should be able to include a threshold value in your upvote, eg. "this upvote counts until the post hits $20 and no more". Even if this is only available to power users, and most interfaces retain simple upvote/downvote UI.