As an amateur FB, Google Ads, and BING marketer, this story fails to make an important point: The higher an advertiser bids, the more exposure the ad gets on the feeds. I would say that the higher price is more due to the Biden campaign looking for that greater exposure than any conspiracy of FB to prefer Republicans.
As for gaming the system, I would say the Democrat strategists are able to target specific ads to specific FB demographics.
Mr. Trump's FB campaign team did a fantastic job of this reach in 2016. I suspect he is now overriding their expertise in marketing psychology. So his FB campaign won't be as effective this time. And with scattered targeting, he probably is getting cheaper ads.