The prices are a tiny bit outdated, but gives you a great idea!
http://golubev.com/gpuest.htm

take the MD5 number and multiply it by 62.63 seems to work well for the 5XXX and 6XXX series, but for the rest, it just gives you an idea in proportion. I'm guessing each series of cards have a number like the 62.63.

another one with a lot less cards is http://pastebin.com/AvymGnMJ

you can then put it here for your average time etc: http://www.alloscomp.com/bitcoin/calculator.php