MH/s data taken from https://en.bitcoin.it/wiki/Mining_hardware_comparison on 5/25/2011 and mixed with price estimates from newegg.com to estimate $/MH/s:
$1.15/MH/s = $735 = 670 MH/s = ATI 6990
$1.00/MH/s = $340 = 340 MH/s = ATI 6970
$0.71/MH/s = $164 = 230 MH/s = ATI 6850
$0.66/MH/s = $236 = 360 MH/s = ATI 6950
$0.61/MH/s = $105 = 170 MH/s = ATI 6750
$0.58/MH/s = $187 = 320 MH/s = ATI 6870
$0.48/MH/s = $184 = 380 MH/s = ATI 5850
(meaning: the ATI 5850 will cost you ~$0.48 for each Megahash per second, not factoring power consumption or number of slots needed in setup)
-jindel