What are currently some of the best ONLINE co-op games, that are also good deals/great value?

One of my good friends just got a PS4 and we're trying to find some fun coop games to play together.

We already have Rocket League.

We're looking for games that aren't too expensive (we're not paying $60 for a game) but are also fun to play online together.

If you happen to know of any sales that are currently going on, please share

Does not need to be a "couch co-op".

Shooters, indie games, etc. All genres are welcome, and added bonus if they're cheap on PlayStation Store.

Thanks for any recommendations!