Most MMOs pay for themselves in sense. Most come with 1 month.
WoW for example, its like 20 bucks then 30 days free. Basically paying 5 bucks for the game.
Some are Free Download of Client with option to buy Retail box for Free game time, convenience of CD instead of a 10GB Download, bonus content and more.
EVE Online - Free to Download of Client but Subscription Required
Lineage II - Same as above.
Aion - Client Free Download (Its on Fileplanet) but Subscription Required.
Just the name of a few examples.