i work at what i would consider a pretty "high end" shop, and charge a lot less than the prices you guys are throwing around from other shops.
we would charge 400 max for that box, probably more like 350 carpeted, and properly braced for 4 12's as long as we are talking simple rectangle ported box....
400 for 2 12's slot port? wow, we would charge 250 for that one.
and with my commission, when i work on boxes, i make like 50$/hr (only when i work on boxes though, which is a few times every pay period), and thats with fair priced boxes, so those guys are just greedy.
what a rip man, sounds like their installer thinks his commission should be what the box is worth.