to be on the safe side, i'd suggest 22 feet... @20 ft you will be down to the nitty gritty. or do what I do and buy a whole spool of 25' and be done with it. shop for a good price online then pick it up at a local auto parts store.
and $500 is crazy expensive, but look on the bright side, he did you a favor by asking so much, now you're motivated to save $400.