Our garage is also our laundry room, so it gets warm in the summer and cold in the winter, and it is not fun to do laundry there. When we bought the house, the previous owner had installed the cheapest aluminum garage door - which was uninsulated. In addition to being ugly, it was a huge thermal load in the garage.
We explored a "kit" to insulate it, but that was hundreds and hundreds of dollars. A new door was thousands. With about $120 of plastic-coated polystyrene (Styrofoam) we were able to insulate the door in about two hours.
The material was bought at Lowe's. We bought 8 sheets for about $13 apiece. We used seven. The foam is coated with foil on one side, and a clear plastic on the other. We used the clear plastic on the inside, as it provided an attractive, wipe-clean surface.
Overall cost: About $120 (we had one panel left over). Time: About 2 hours.
The garage is already quieter and cooler. We have an old window unit in there to keep it cool. Great for working on your car!