I'm not sure if it's still there, but there was a thread on WW about making your own welding extension chord. There was ton's of debate about thickness, but using a 10-3 bulldog power chord is a cheaper alternative. Just change the plugs to match what you need.
50'
http://www.northerntool.com/shop/too...4937_200314937
100' not sure if 10 gauge is thick enough for 100'
http://www.northerntool.com/shop/too...4939_200314939
50'
http://www.harborfreight.com/general...ord-93670.html