It's been a safe assumption ever since PlayStation Plus first offered free games in its Instant Game Collection that it can't be free for Sony. There isn't much point for a developer to distribute their games for free apart from exposure and temporary appreciation from fans, unless there's a substantial incentive.
Now this transaction has been made crystal clear. Brian Silva, PlayStation manager of developer relations, revealed that Sony will have "buyouts" of some games to be free for a month on PlayStation Plus, during a discussion with developers at Indiecade.
The exact amount that Sony gives developers wasn't reported, though that may not diminish the feeling that these free games are a part of some dark business plan. But it all makes sense: The developers receive an assured amount of money in the bank as well as advertising for about a month, while Sony bolsters the number of subscriber for PlayStation Plus. It's a win-win feedback loop, and the savings are passed onto the consumer who, as Anthony calculated, saved over $1800 in games in a single year.
The more, the merrier. Besides, it will only make PlayStation Plus that much more attractive for PlayStation 4 and pressure Xbox Live on Xbox One to have free deals of its own.