A lot of garage doors are insulated with styrofoam panels which are just glued on to the door so they are very lightweight and can easily be replaced by homeowners.

Styrofoam panels are available at nearly any hardware store and the job would probably cost less than $500 - $800 and a weekend depending on the size of your doors(s.) You’d just need a few simple tools and adhesive and you could do it yourself. I’m sure there’s a YouTube video on how to do it.