I did the math the other day, and I think $19.95 is the perfect price for the casual game/gamer.
For instance: I have a game club membership because I find myself wanting more than 4 or so games in a year, @ $20 a peice, 4 or more games is $80+ bucks.
My club thingy gives me 12 games for about $84. This is a great deal for me, (but even I find myself with 4 or so extra credits at the end of each year, but i give these as gifts and it evens out)
If one doesn't play this many games..... Maybe 2 or 3 in a year, then they should just pay regular price and save the $25 or so.
And the conscious consumer would gage these numbers and do what's right for them. If the price were any higher you would be pushing away a more casual audience, and if it were lower those that sell subscriptions would probably lose subscribers!
Buying subscriptions is something that will only appeal to a certain kind of gamer.