[akadruid]

The cost model to this is proven - there are MMOs that operate this way already, with monthly subscriptions but no upfront cost. EVE Online works like this - you start with a free trial for 21 days, then pay for each 30 days of access to the server. There are optional boxsets, but with those you are essentially paying for the included 60 days of access and some physical items, rather than the game.