The Rockies Parking Lot is surprisingly cheap for a MLB team at only around $8. The only downside is that its very difficult to get in and out of this parking lot. There is also street parking a 3/4 mile down the road and then you don't have to worry about the backup after the game.

Unless you want to park across the street from the stadium, you should be able to find parking for under $10 for most games. Surface lots and garages are aplenty off of 20th St. The Rockies also have a stadium owned parking lot just north of the stadium.