If your dealer can get the configuration you want from his distributor, you'll get a nice, low price on it, because distributors get big discounts for buying thousands of rifles at a time, and they pass on some of that to dealers (i.e., the retailers).
If you insist on a specific configuration that isn't available from the distributor, the dealer can only order it directly from Bushmaster, and will be charged RETAIL price for it. The dealer will then add on to that to cover costs and some profit.
The reason the dealer doesn't get the big discount the distributor gets is because selling single rifles is a hassle and an extra cost for a manufacturer (which is why most companies don't do it at all), and because the distributors have contracts with the manufacturers ensuring the lowest price. If the dealer sold rifles separately at distributor price, the distributor wouldn't be able to make money, and would stop carrying that brand, and costs would go up anyway due to Bushmaster having to act as a distributor, and factor that cost in their pricing.
Buy a complete lower from your dealer, and order the upper configuration you want directly.
Since almost any dealer AR configuration (except a few M4 models) comes with an HBAR, and I have NO use for HBARs, I always buy separately.