If you have a garage, you can do the fitting yourself. Usually you'll just need to trim and cut, which you can do if you have one of those small blade grinders.
I bought an amuse kit 2 months ago. I paid $300 to have my front bumper fixed and fitted (because it broke in several location during shipping). I need to take the rear bumper to the same guy to do the fitting, but he already quoted me $300 for it.
And he will charge me $700 for paint, so in all it will come out to $1300 for fixing/fitting/painting the kit. This guy worked on my 350z body kit so I know the quality of the work he does.
I just cannot believe people would pay 4-6k+ for fitting and painting 2 bumpers. You can paint a full car for less, and that's more work than just fixing 2 bumpers. A lot of people get quotes at high end exotic body shops that mostly work on super or luxury cars and they are charging you for their reputation, not the actual work.
I support small businesses, so I pay less
Edit: I am not doing side skirts, because they suck and make your car too low, so that factors into the low price, but even with side skirts, 4-6k+ is insane. I am doing the wing but no fitment is needed, I will get it painted and I will install it myself, so I'm only paying to have front/rear bumpers and wing painted.