If you are getting Torklift tie-downs for front and back the dealer's price for parts and labor is right in line. It depends on what components will be used, in particular the turnbuckles.
Front and rear Torklift tie-downs $497
Fastguns - two pairs - $440
Lance 7-connector harness $48
Battery isolator or dual battery switch $28
Fuse - $4
8 ga battery wiring and connectors for harness - $15
The unknown is the time to install the Torklift tie-downs on your truck. With some trucks it is easy and could be accomplished by an experienced installer in a couple hours or less. With my 2011 GM shortbed truck it took me 8 hours which included having to saw off sections of plate, grind down corners, and repaint the brackets to get them to fit properly as the sizing for the front pair caused them to hit the frame before the bolts could reach the holes in the bed and one of the rear bracket had its corner pressing against both the rear brake and the fuel lines. Torklift makes a very good product but the 2011 GM kit was not something I would recommend to anyone I liked.
I would guess that there is about 4 hours of total labor so the $300 is reasonable. Best part is it is locked in regardless of the proplems the installer may run into with your truck.
Bottomline the dealer's estimate is fair if you are getting these items and you can get a detailed breakout of the tie-downs, turnbuckles, and other parts to be provided and check pricing out for yourself.
The dealer is not paying "retail" for the tie-downs and turnbuckles so you may be able to negotiate on the installation. I did this with the 845 where I got the dealer to sell me the AC unit at his cost and install it for free. Paid less than $400 for a new 9.3 Coleman unit installed on the camper. Dealer sold the camper and was not out of pocket for the AC so it was a win win result.