it depends if you encase it and if you run it into the cabin. uncased and installed just under the hood you can complete one for under ten bucks. add a case, anywhere from $3 to $5 depending what you buy, run it into the cabin and youll need about 8 feet of wire to do it CLEANLY, so add anothe couple bucks.
granted, most people already have some extra wire and some random ass enclosure they can use as a case, so these may not be a factor at all if you are resourceful
