I don't think the price you were quoted is too out of the ball park either. I haven't gotten any quotes for my '15 Yukon XL Denali, but I did get some quotes to do a complete color change vinyl wrap on my previous '09 CTS-V Sedan and it was about $3700-$4000.
Take into account you're talking expel "satin/matte" paint protection film which will be more expensive material compared to just colored vinyl add in the overall size of a Yukon compared to a mid-sized sedan and I'd think $6,000 is right in the ball park.
A complete car wrap is a lot of work, tons of prep, especially with a paint protection type film. I would assume the installer was figuring a full extensive clay bar, paint correction, and prep before the expel installation. That alone can be $350-$600 depending on location and condition of your paint.
Expel makes a great product, I'll just say if you're going to go for the satin/matte finish and drop that kind of coin on it just be sure it'll be something you're happy with for at least 5+ years (it could last longer?). After that the stuff is pretty easy to remove with some patience and a heat gun, then if the product did it's job you'd reveal what would essentially be close to "brand new" OEM paint.