Doing it yourself is seriously super cheap if you're willing to DIY it a bit instead of using pre-cut-for-a-garage-door kits. I have a small one-car garage and by cutting a foam insulation panel myself and installing it with foil tape I did mine for like $30-40.

The trick was doing each section with two pieces, side by side. Each one scored and snapped horizontally so I could insert it slightly folded and when I pressed it in it locked into place against the top and bottom of the panel. On my door there's a small void one one side of each panel, so I slid one piece into that, then inserted another next to it.

It looks great. It works great. Makes the whole garage quieter, even. Dampens rattles when the door is moving. Weighs maybe a pound or two.

Edit: If you're looking to keep a garage warm, it's also well worth checking and possibly replacing seals. It's not very hard to replace the bottom seal, maybe even install a strip on the ground below it. It's also generally not hard to replace the exterior seals on the top and sides. I've done all of the above and having my garage well-sealed has worked wonders on its temperature and keeping out traffic/city noise.