Join Date: Mar 2017
Location: Just west of weird.
You're paying for time and effort, mostly. Also, because paint shops are likely limited via contractual obligations to who they purchase supplies from, they're likely having to purchase a larger quantity of paint than what it would actually require to paint your spoiler. The spoiler has to be cleaned, sanded, primered, sanded, base coat, sand, 2nd base coat, sand, 3rd base coat, sand, clear coat, sand, 2nd clear coat, sand, 3rd clear coat, sand (at least I hope they're doing 3 coats of each) and then the paint has to be cut with a polisher using a heavy cutting pad, then a lighter cutting pad, then a finishing pad.
You're looking at several man hours of work for your spoiler, and that's what you're paying for. Most shops charge about $80 - $125 per hour for work, so this seems 100% legit.