Bushmaster's emphasis seems to be on selling lowers or complete rifles.
Around here the gunshops will sell anything they can make a reasonable profit on. I only see complete rifles or lowers from bushmaster. I asked why and was told that they "want too much wholesale for the upper alone."
My guess is that bushmaster sells enough of them direct from their website and makes a larger profit that way, since a transfer is not needed for an upper. I a ked my gunclub's FFL about it and he told me that he can only get the price from the website for the upper, but can get a deal on the complete rifle or the lower alone, but even he, who gets good deals on most firearms has to charge about the same price as the gunstores after shipping and transer fee ($25, plus a $15 tip for his time).
Rock River on the other hand can he can sell for something of a deal.
I think to get a deal from bushmaster you have to buy a large quantity of complete rifles and lowers.
I have yet to see a deal on a bushmaster upper anywhere.