I paid ten bucks a month for Asheron's Call, and that game kept me playing for about three and a half years... I figure, I usually buy console games for $40-$50, and they keep me entertained for a month or two, and I spent about a total of $450 on Asheron's Call and it kept me entertained for over 40 months. The money to time ratio isn't bad at all.
There's always online PC games, but all of the massively multiplayer games have monthly charges. Regular multiplayer games like Counter-Strike and BF1942 are cool, but it's pretty much the same thing over and over... There's no real feeling of progression through the game, just logging on, killing some people, logging off. The satisfaction involved with leveling up and earning new equipment and spells, etc. in a MMORPG is much greater than shooting guys with rifles over and over, in my opinion.
Then there's games like Neverwinter Nights, which has free online play but is far from massively multiplayer. I would highly recommend NWN if you don't want to pay monthly for an online rpg, but it seems as if Bioware is planning on releasing expansion packs for the game every other month now.
Seriously though, I haven't even bought shadows of undrentide yet, and they're already talking about the next expansion pack. Most of the good servers require that you buy the expansion packs to play on their servers, so you're going to have to shell out extra money after purchasing the original game anyway.
If you don't want to pay a monthly fee after buying a game, that's fine. But the monthly fee is well justified. I could go into a rant about the fact that companies need to constantly maintain their servers and keep a staff working on the game 24/7, but pretty much everybody already knows that. When you're paying the monthly fee, you're paying for an extremely immersive online experience, much more than any of the games that offer free online play. You're paying for constant updates and bugfixes, new weapons and armor, new spells and skills, new areas and monsters. Unless you're playing everquest, most MMORPG's offer new content with their patches that most games would require an expansion pack for. Everquest is the exception, because they enjoy squeezing every last drop of money out of their subscribers by releasing an expansion pack for every single addition of new content, and actually getting away with it, but I won't get into that right now.