Silver Subscription for XBOX360 LIVE will just be chat rooms, and parlor games (eg. checkers, chess), while the Gold subscription will be like the current Xbox Live service.
Dustbunny also made a valid point, the level of service depends on how much you pay (usually). Most CS servers are paid for by clans, and/or generous individuals that love the game. Most MMORPGs also require a montly subscription fee.
Even on Xbox Live, the amount of service you get depends on the game. Some games have more online functions than other games, it just depends on the designer, and publisher.
The more people that play the game, the less the subscription cost should go down, because as you increase the amount of cash provided by players, the overall price per player can go down, provided the fanbase dosen't grow so fast that the game can't support itself. It also depends on how hardcore the gamers are, if they play more of the game, it takes up more resources, than if a casual player would join the service (eg. server), every once in a while.
Now You Know