When I was commuting out of DEN I used to park free at RTD's Park N' Ride at 40th St. and Pena Blvd. and ride The AB, AF or AT bus for $6. If you want free covered(hailstorms, you know) parking, park at the Stapleton transfer center and ride to DIA for the same price but slightly less frequent service.
Outlying on airport parking is $5/day and a free shuttle to terminal.