Hey, I like CCP's solution to this, in EVE, you can buy extra months of subscription, and sell them to other players, on the market, for Gold (ISK). I play the game for free, because I have enough isk to sell to folks who want more of it.
Eve's economy actually works pretty decently, dudes get alot of use out of having extra isk, they can fly bigger ships, gamble more, pay folks for whatever they want. I always suggest to my friends that they buy three months of game time when they start playing, 1 month for themselves, and 2 months to sell to the market. Everyone gets on a nice, even playing field pretty quick that way, (and it's still cheaper than starting alot of MMO's).
To ramble off topic for a while, market manipulation is incredibly easy in eve, I play for free because I spend about 3 hours a week looking over trades in three regional markets. I had to put in a bit of work to get enough money to afford it, but the cash I have is still chicken scratch (barely floating a billion isk, and most of it's tied up in one thing or another)