Actually you can run it for 120 days using rearms:
http://www.mydigitallife.info/2008/1...7-to-120-days/
Hacking the registry key that keeps count might work forever as well:
http://www.mydigitallife.info/2010/0...s7-rearmtask4/
But, this stuff usually gets fixed over time, so I'm not sure that the unlimited rearm thing still works (or will work with SP1). The 3-times rearm thing is legit and intentional, so you've definitely got at least 120 days (assuming you did it on the last day of the 30 day trial each time, since left over days are just lost).
Edit: Forgot to mention, if you're looking for a good price, get together with a friend or two and buy a family pack key for Win 7. It's like $150 for 3 keys. Or you can always find someone in college to buy you the $30 student upgrade version if you know how to make it into a custom iso that will install clean instead of just upgrade. Lotsa options. ;)