£12.99 a month - with a free 7 day trial. They have quite a backlog of PS3 games that you can stream on your PS4. I've had a look at a few of the games available - Bioshock 1,2 and Infinity are the stand out titles for me. I've started the free trial, graphics haven't been updated but it is all good fun.
I'm unsure how often they upload new games, as £12.99 seems fairly steep.