According to partpicker, an AMD FirePro V9800 will set you back 2.8 grand.
http://www.superbiiz.com/detail.php?...T-FPV9800&c=CJ
A V7900 is "only" 680 dollars
http://www.superbiiz.com/detail.php?...T-FPV7900&c=CJ
Don't know about the relative power of those cards though. I'm sure that Cyan knows a lot more about it than I do.