Doesnt cost any more than a similar 5.56 setup. Radical firearms or hardened arms complete rifle should run you $550-$600. i.e. Radical firearms complete rifle http://www.radicalfirearms.com/product-p/rfrifle.v2.16fgs-mil300.htm
The 300 and the 5.56 are the same price. You could also get an ar pistol in a 10.5 barrel config in 300blk and have a nice compact rig.
I can testify for both companies. I have had both and they were good quality and shot good.