There are several games such as EVE online, puzzle pirates etc. Where you can buy monthly sub directly from in game market/other players depending on market price, only by spending in game currency(credits). The way it works is a player with real money buys an item which is consumable (30day subs card/badge/license etc) and sells it on the market where other players can buy it.
It works good both ways due to simple fact:
1. people like to stack in game money
2. Not all players can use a credit card due to young age or/and dont have enough money to buy sub. In some areas 15$ is a big deal. In my country for 15$ i could watch 3-4 movies in cinema or play bowling for 5hours or buy ~20 bottles of beer.
3. There are plenty of people (look at points 1 and 2) which would be willing to buy such items and it would naturally bring more dedicated players and players in general to the game. And developers would make more money by doing so.