£250 for yearly access with DRMs on the games to stop them being updateable after lets say... 18 months? £500 for infinite access, or £20 a month.
I don't think that most people would use it enough to qualify spenidng much more.
and!
it would have to complete with things like lovefilm who offer game rental, up to 4 titles at a go, which is more than you would ever use!