the cheapest and lowest sound quality would be to run your mixer 2 channel stereo mix to your PC's soundcard line-in. You can't adjust anything later, but with a little trial and error you can get it onto a cd. Wouldn't be pretty, but it'd be.
Spend $500 on a MOTU / Presonus 8 channel fire wire interface, and you could ditch the sound card with its 28 cent AD converters, and record 8 distinct tracks. 8 channels is pretty tight for a whole band, ie 4 on drums, 2 guitars, a bass, and vox. If you get 16 channels, you can get more drum tracks, get a di and a mic on all the bass/guitars, have a few vox, a room mic, etc..
So, you could do it for practically free, or you could spend a ton of $. Whats your budget and requirements?