Hitch installation cost for 2017 CX-9

I bought a Curt 13284 trailer hitch, along with the Curt 56338 4-pin trailer wiring harness. The local U-Haul dealer quoted $175 just for the hitch installation. However, to have it installed with the wiring they quoted $356 total.

Why does the wiring cost so much extra on top of the hitch installation?
2016 CX-5AWD GT+Tech/ 2018 CX-9AWD GT
Wow, that's a pretty penny for just wiring. Connecting the wiring is pretty easy, like a 15min. job...I wouldn't pay that.
Mazda CX-9 Grand Touring 2016
You do not need a trailer specialty place to install the hitch & electrical connectors. Call your favorite local mechanic or anybody handy with tools. You can find videos of the procedure on etrailer.com or maybe youtube. Do check other hitch installers, but they probably will charge extra anyone who brings in items not bought there. The mark up for parts is part of their essential business model.
They are telling me that there is no prewiring In my 2019. It that true
Here is a video of wiring install. Its pre-wired for the standard 4pin. I dont think it is pre wired for a 6 pin though (can somebody confirm).

Note that the video goes all out with routing the cable to the outside, mine just sit inside the cargo area and as flat portion to close the tailgate on it. I would rather wire it to the outside like they did in the video but thats a project for a later date.

2009 Mazda CX-9 GT
Hitch install is 30-45minutes.
T-one wiring install is 15-20 minutes.
7-way wiring install can be 60 minutes or so.

Take your local shop's labour rates and do the math. The prices you were quoted are ridiculous. Some shops will overcharge when they don't do a certain type of job all the time, because they know it will take them longer than the book time. Your quotes appear to be just that.