What would I expect to pay for this upper reciever from Bushmaster, DPMS or Olympic provided they offer this particular model?

I don't know.

You should check out Rock River Arms:

http://www.rockriverarms.com

They offer a A3 upper assembly, with a 20" Wilson stainless steel 1:8 twist. match chambered barrel, complete with bolt, bolt carrier, and charging handle for $420.00.

I know it's not what you asked (Bushmaster, DPMS, or Olympic) but you ought to give these guys a call.  Seem like a helpful bunch - they'd probably flute the barrel for you for a reasonable price . . .

Quoted: What would I expect to pay for this upper reciever from Bushmaster, DPMS or Olympic provided they offer this particular model?

Olympic has the high end SUM stainless steel Ultramatch K6.  Free floating, etc. Al handguard.  1x8 is not extra charge.  Broach cut vs. the others cheaper button cut.  $636.00

Fluting will cost you roughly additional $100.00.  Fluting loses about 12-14 ounces of weight.

You can also get a cheaper upper that is more comparable with the rest of the companies, but not as accurate.

mark