The other game that you can play for free is Eve Online. Not sure what the value is, but provided that you can generate enough ingame currancy (or ISK), then you can pay your monthly subscription using that.
Not sure how you do that, but i know that there are a few member here on TF that has done it and only had to pay for the first month's subsciption.
The only problem with EVE Online is
1. Steep learning curve. The tutorial is about 2 hours. Sit for it because if you start paging every one "How do I fly my space ship?" you will have the rest of the server telling you to "Sit the tutorial"
2. Death is very unforgiving in this game. Cloning and Insurance is your best friend.