It's impossible to determine the profitability of the PSN... well, Sony know but they ain't telling.
But there is another definition of cost we can consider, and that is the diffeence between the total profit with charging for it and without.
So making some fairly big assumptions
All PS3 users will subscribe
There will be no costs with marketing this system, or profit taken by retailers
This does not decrease PS3 sales at all
27 million consoles @ $50 per console per year = [edit]1.35 billion per year, which is approximately equal to the divisions annual losses[/edit]







