Most traditional games use a subscription or pay for software scheme, but that is not the only successful way to make money for the gaming companies. Case in point is a little game called Project Entropia by MindArc
The premise is simple: have a game where players can trade real money for online currency and the reverse. The company makes money whenever a player deposits and looses it when a player cashes out. The marketing strategy is very similar to an online casino.
Players start with no money and nothing but clothes and a debit card to their name. They have abilities, but all but one of them require tools purchased from other players or from an NPC. The only thing a new player can do to make money besides depositing is harvesting mind sweat from local creatures. This usually ends in two hours of playing and sweating before the first few bottles are collected. These items sell for ridiculously low amounts, as anyone can create the sweat by sweating creatures and if the price is too high, market factors will balance it out.
Beyond the bad initial income, players will loose money if they do anything and sell to the NPC traders. The wholesale prices are so low that it is almost impossible to turn a profit that way. Any profit must be made from other players. The most successful players haven't made that much money off of MindArc, instead they've made almost all their money off of players in one way or another.
The model has successful players leeching off of new players, or players who are bent on depositing and having fun. From a playing perspective, the game is free, but hard to play if you don't deposit. From a company perspective, you will be making much more money than you're paying.
Some suggestions, though, for this model. If you choose to impliment it, make sure to strike a balance so that it is more profitable to sell or buy from a player than a general store, but some specific and neccesary items are only purchasable from the general store. You want to maximize the drains on the economy, so that more money is removed from the economy and into your company's pockets.