The Rockpile in center field is the way to go for an experience where you don't have to spend money. Kids 12 and under and adults 55 and older can see a game in this section for only a dollar, repeat only a dollar. People from ages 13-54 can see a game here for four dollars. There is also a section in the very upper deck which puts you exactly at a Mile High but you are far away from the action. If you do want the best seats go for first base line and pay anywhere from $30-$60 bucks.
Coors Field offers seating options for any budget. For just $4, you can purchase a Rock Pile ticket at Gate A. Those tickets go on sale 2 hours before first pitch. Rooftop tickets are a little more expensive and will get you a reserved seat in the rooftop section and a small food & beverage credit that can be used at one of the multiple vendors in the rooftop section. If you're looking to spend a little more, often times you can find seats for about $40 and those will get you in the 100 level down the 1st and 3rd base lines.