An out of warranty repair is free if your Xbox is suffering from RROD or an E74 error. Other out of warranty repairs do cost money.
Go to xbox.com/support and select repair your console on the left menu. After that, follow the steps to get your xbox registered, and they will give you a cost assessment. I recently sent back my elite that RROD'd and got a brand new one back, manufactured only a few days before my package came in, which would explain why it shipped from out of state.
Don't fuck with any of the "fixes" for RROD, since MS will fix it for free out of warranty for 3 RROD or E74.
Edit for above post: Thermal Paste. If you're going to replace the heatsink and paste, just use Artic Thermal Silver.
http://www.arcticsilver.com/
Also, if you open your Xbox, while you might have had a free fix, you WILL ruin your chances at one from MS. You also run the risk of completely bricking your system with static electricity, or just fubaring the shit out of it with loose screws/bad screwdrivers.