Some MMO's use a F2P (free to play) model.. which is ok if you only play a little bit, the way they make their money is to offer in game items for a fee
I played FlyFF previously with renta which was free, but all the cosmetic items and items required to make upgrades etc had a "gpot" cost.. you could buy x amount of gpots for a few dollars and exchange them ingame to get the item you wanted
In the end tho you end up paying more than say a wow subscription if you want to make any real progress