I don't know of a particular site with such coupons. However, USAParking (both open and covered) is a few dollars cheaper than comparable parking at the terminal, and with AAA you get an additional 10% off; $1/day off coupons for them used to be pretty easy to find. They pick you up and drop you off at your car and will help with luggage. Coming from the north, it saves a toll booth also. Their vans seem to be running a little less frequently now, so I would assume a few minutes wait on both ends.
On the airport property (and maybe nearby also), Pike's Peak parking is probably the cheapest (uncovered only, AFAIK); there the buses pick you up at ends of aisles (luggage assistance is rare in my experience).