While the prospect of replacing that door on your own will definitely save you money, it also puts you and any other individuals at potential risk. Those people will be needed simply because the average door weighs anywhere from 150 pounds to one ton. Failure to maintain that weight can lead to tragedy.
In addition to the door weight issue, having the proper tool and the training to follow instructions to the letter is important. For example, tightening a door with torsion springs requires using winding bars and use of a screwdriver here could result in injury.