WoW would have so many more players if they lowered the price. Hopefully Rift is smart and makes it cheaper.
It's really not that expensive.
Let me give you a run down of why it's a better deal than most games now.
Alright, so most games now cost 60-70 dollars when they are brand new. The average playtime you get out of most of these games is 10 hours. So you spend 60-70 dollars for 10 hours of game play. Perfect example of this is Fable 3. On top of this, most game developers don't support their games much after they release them, probably due to the fact that they won't make much more money on them after the first year. Good example of this is Oblivion (for PC). The community has patched more of that game than Bethesda has. Lastly, most of them time you get charged for additional content, such as DLCs, which maybe give you another hour or two for 10 dollars.
So now, take an MMO with a monthly subscription:
You pay the upfront cost for the key, which includes your first month of game time. Say that, at minimum you, play 2 hours a day during the school week and another 5 a day on the weekend. That's already 20 hours for the first week. You'll probably play the game for at least a few months if you like it enough, so that could be anywhere from 240 to 320 hours to over 3 to 4 months, respectively. Now, looking at this from an overview, if the game cost 70 dollars initially, and then 15 dollars each month for 4 months, you've paid 130 dollars total. This is 40 cents for every hour you played. Taking the other game, at 70 dollars for 10 hours, it costs you 7 dollars for every hour you played. Even if you get 20 to 30 hours of time out of it, it is still 3.50 to 2.30 dollars per hour. Also consider that you may only play your new game for a week, and say you got a new game every week for something to do, that's now 280 dollars a month.
Along with this, content for MMO's is generally FREE. Usually expansion size content is not, but generally it is. For WoW you get new tiers of content every 4 months, and those tiers last for usually 4 months. One game in particular charges 20 dollars a month, but they've stated every expansion that they release will be given to their users for free.
Finally, these games are constantly patched and updated to ensure a good gaming experience for their users.
Blizzard knows this. They make more money on WoW in one year then all the console games combined sold that year will make.
EDIT: Oops, messed that calculation up a bit, included the 15 dollars for the first month "free" with the total. It would be 115 for 4 months at 36 cents an hour.