MaxxFan run around 250 each, give or take. Toppers, highly dependent on size etc. I am not familiar with a 3150BHD, although I am guessing larger slides.
That quote does seem high, but not total outrageous. I assume this is on a new rig? If so, negotiate with the dealer. We managed to get our selling dealer to throw in our 3 toppers. It took some heavy negotiating, but my wife is a master.
2015 F-350 SRW 6.7 PSD
2014 Crusader 295RST
1 Fantastic DW
1 Amazing DD
1 Beagle that will be missed (RIP Zeus)