Very good question.
The answer to your question is:
"Because people will pay it."
If enough people stopped paying for it, they would have to either increase the (already big) advantages or lower the prices.
As long as people are willing to pump money in at the current prices, they sure as hell won't lower it.
That is a bad answer. That is if you knew anything about economics. Right now there are many games out there that are cheaper and provide a much richer and complete gaming experience. Runescape is an example. Kingdom of Loathing is another.
High prices work well with items of finite availability but work horribly for items of infinite availability. Premium costs them nothing to make. So they can afford to price it at any price they feel like.
The Upside of pricing high is that you get a premium price for your product.
The downside is that you limit your customer base. If people feel they cant compete in a game they will likely move onto another.
By offering the game at half the price you lose money from the initial client base, but you also gain many new clients. With a new game it is even more important to offer low prices as you want to retain as large of a client base as possible.
By offering the game at a quarter of the price you lose even more money, but again, you gain more clients which increases money. Another advantage to having prices this cheap is that more people will play premium on more than just one world. This increases the client base even more, giving a much more competitive game experience.
but blah...i know nothing...
Yogi-