According to the UK-based Push Square, the retail value of all the titles in the PlayStation Plus subscription for 2014 was $1,349.29 US dollars.
This data factors in every title on every platform, so to even theoretically get the full value, one would have to own a PlayStation 4, PlayStation 3, and PlayStation Vita. However, even owning 5% of the content offered through the service would produce a benefit; the subscription only costs $50.
Joystiq notes that the average metacritic score for the games in the Instant Game Collection is 79, which means Sony isn’t trying to pawn off some crappy relics, but rather giving players the games they want to actually play.
So go ahead, spend the $50, it’s still the best deal around.