Your prices are outrageous, you charge more for 1 month rental of a 7770 than you can buy one for ($92 on newegg). The 7850 can be purchased for less on ebay than your one month rental... When people buy video cards they have the ability to resell them in case the coin crashes, if you want this to be anywhere near successful, you're going to have to charge 10-25%/month of what the video card costs, not 125%.
These prices are based on the UK market prices for example a 7770 costs $169 dollars here in the UK so I didn't realize that for US users they are extremely expensive. I will try to fix the prices or I might only offer the 7970 for now.