I always use hotwire.com It's $16.95 a day for a Pontiac G6 or compatible. They have cars as low as $13.95 but that's for a really dinky car. The G6 is mid-size category and the smallest I would drive. I've rented from them on every trip to Denver. They have a shuttle service from the airport to the rental pickup/drop off location. Pickup and drop off is a simple process. No hidden costs either. I have a trip scheduled for April 9th pickup and April 13th drop off and the total price is $92.98. That will be the total in the end unless you add insurance. Check with your own insurance company and see if you have coverage if you rent a car.
You doing anything special to get these prices? I am going to Denver the beginning of March and the cheapest prices on hotwire.com are like $24 per day for economy cars. I checked on prices a while back and could not find anything cheaper than this.