When I have considered rewarding downvotes in the past, my idea was to reward any vote which moves the payout toward its eventual result, but not those which move it away. So for example, if there is an upvote to $10 and then a downvote back down to $0, the downvote would get a reward but the upvote would not. Yes, this means that the downvote got a reward even when the post did not, but this could be justified in that the downvote saved the system money (paying out on something which in the end, was determined to be worthless).

There are no doubt numerous complications and this may not be feasible at all, but I thought I would throw the idea out there again.