The GMTU kit or Stage 1 kit as some call it, is outdated. An aftermarket tune is the way to go now.
The Stage kit was fine way back when the SS was new or almost new because GM would let you retain coverage of the factory warranty.
One reason the kit was priced so high was because GM was "pushing the limits" so to speak, and needed to charge a little more to cover any warranty issues.
Now that none of the SS's have a factory warranty left, there's really no need to consider this tune.
Besides. Since the SS is going on 10-12 years old, there aren't many GM dealers that are still familiar with installing the Stage 1. Most owners that had it done, did so years ago.
As far as what the dealer did charge, the book called for 2 hrs labor. That charge was for the dealer to do the install. That's about how long it took the tech to do mine. Physical labor wasn't much time but getting GM on the line and downloading the reflash took over an hour.
$95 x 2 hrs= $190 is what I paid almost 10 years ago for the install.