I made a 25' as I recall for my Miller 210, and if memory serves me correctly, I got 30' for the price of 25' at Home Despot since the guy didn't want to leave 5' on the reel.
Just make sure you size it for the circuit, and you'll be OK. The wire will be the costliest part due to the copper in it, and I wish I could remember what I spent to make mine.