If you are looking for something plain, Home Depot sells 14 gauge for $0.30/ft. and 4x16 gauge (or about 13 ga.) for $0.24/ft. They may also sell it on their website though I have not looked.
Markertek.com has monster 12 gauge for something under $1/ft. It's hard to find, do a seach on the word "gauge" and it turned up in there. Not sure which line it is, but it's a pretty good price even for their low end stuff.
Edit: OK just checked. $.45 per foot for 12 gauge. Minimum 50 feet. Click on audio cables, then in the left search on gauge, Monster speaker cable is the firs item that comes up. 14 ga is $.36. Marker tek also adds $3 to orders under $35. But they ship very fast.