So I saw the pay with a like, I thought I had commented there with this idea, but seems to have possibly been removed. Anyhow.. I see the pay with a like is more just a typical content locker for getting content. It could be used to unlock a coupon to use in Prosites, but I think a much better way to go on a similar idea would be to branch off and use a "like/tweet/G+" to actually level up in prosites. This will cover off many of the plugins that already work with Prosites. This will be a very good way to not only make Prosites more robust in monetising a site, but also get more people using the social share to benefit themselves with a site that wants to do things like this:
Prosite setup with $2/month to remove ads or tweet (predetermined/configurable message) to remove ads, $4/month to have level 2 Themes available or Facebook Like/share (predetermined/configurable message) to enable Level 2, $6/month to have Level 3 for Google+ share....
Possibly another share/like to enable a free online store/site at signup. So in fact you could earn quite a bit of social sharing on signup and creating a store including the extra ones to enable features for free through share/like.
This would all do 2 most important things, save your users/member some money thus creating much more advertising to get more users signing up from the users friends looking at what is going on. I am quite surprised that there is not already something like this on WPMUDEV.
So if you love this idea, feel free to throw some rep points my way as well as vote this in. My thoughts are this would be a powerhouse of a plugin/extension to Prosites and really tie in some very important features.