If you have so much trouble to find five good posts a day, have you considered white listing curated users and limiting everyone else to one upvote every 5-10 windows?
That would improve profitability for those found to create quality, and limit abuse from the rest.
Thinking about ways to deal with the garbage spam, there is just so much of it that it is endless. We don't want to use whitelisting as it will curate the same people over and over and we try to feature as many unique authors as possible. For a while, we were having the same authors show up a lot.
I do understand wanting to make the service available to a wider audience. That's why I suggest keeping it open, but limited to one upvote every 12 hours (with a white list that removes that upvote restriction).
Another alternative would be a white-list for higher bids, so that low quality authors are limited in the upvote they can get. This would make your problem worse, though (more low-quality posts at the lower limit).
Have you done any metrics around avg. bids by quality level? Having some insight into whether the bidding patterns are different for different quality levels might make it easier to make a plan that boosts quality overall.
Not sure how, but this landed in the wrong post. Was intended as a comment about the @buildawhale changes.