$15 really isn't all that much, unless you're on an allowance from your parents. For people actually working jobs, even a part-time job at a fast food place (not me, thankfully), it's rather minimal. However, I'm still reluctant to pay, because I haven't found the gameplay itself to be nearly as enjoyable as many of the single-player experiences you can get.
Far Cry was mentioned above...it was good for 25 hours of gameplay or so even a single time through, so playing 90 minutes a day Monday-Thursday (weekends are for friends and going out, not sitting in front of a computer getting a CRT tan) means it took me about a month to finish the game. And, if I want to stop playing and pick it up at any time, I can without having it cost me a dime. I don't have to wait for friends to come online, nor do I have to deal with other players exploiting, griefing, or making nuisances of themselves.
While I have no clue how well it'd work, I'd love to see a distributed-server-based MMO game, even as a limited experiment. Give individual admins the power to shape a part of the world, and should it be deemed a good-quality, consistent addition to the world, a master server would attach it to the network. The traffic of people to the server would determine the chunk of the subscription fee, which would cover the rising server costs and reward those who make an interesting part of the game world. Players also get the option of choice; if they don't like a section of the world, they don't go there, and it withers and dies, while those they enjoy grow and flourish.
There's also a much smaller outlay necessary for the initial developer. A few Lego-style set pieces and good terrain generation features, along with the ability to define more via a 3ds / Maya / Softimage / preferred modelling app plugin, would give those interested both a good starting point for continuity's sake and the ability to extend the world as far as their imagination and talent allow. Hopefully, these same creators would allow some of their art/models/terrains to be reused by others, and at the same time be allowed to suggest modifications to existing resources or rules. Think of it as a US-style representation scenario, except even more free. Imagine if the admins on your favorite multiplayer server could suggest additions and changes to the game overall, how much better it could be. Of course, this is all pure imagination, and I don't have the bursting-at-the-seams bank account to fund even the inital costs of a project like this. But I'd certainly be interested if anything resembling it popped up in the market.