Does anybody have any idea how the cost of a commercial-type rolling steel door compares to the cost of a conventional sectional garage door with tracks? Say, for an opening 18-20 feet wide, and 7-8 feet high (the lower numbers are minimums, the higher numbers are values that it could be if it meant saving money by using an off-the-shelf size).

A normal residential garage door won't work, because this is for a SECOND door in the SIDE of the garage, next to the parked car. Presumably, the tracks for the main/front garage door would get in the way of the tracks for the side garage door. The general theory is that a rolling steel door only needs about 2 feet of space behind the wall where the door is installed... so with a 14-foot interior and 9 foot door, there should be enough space to fit the entire side door to one side of the front door's tracks.