Lexan can be pretty pricey. I think I found a 4x4' sheet for < $50 4 or 5 years ago. If you have to buy a 4x8' sheet, you may be well over $100.
Like many things, some people have skills to figure it out and do it themselves. Others not so much.
When I think about paying a dealer $125/hr to do something I can't do, this doesn't seem all that outrageous for a mom & pop garage business.
I'd guess they got some kind of patent for how it attaches to the door. Could they defend the patent? Probably not. But maybe they were planning on going on Shark Tank, and you know the sharks would have asked right off if it was patented.