Gunsmithing Cost to rechamber a barrel?

I realize 1:7 is stupid fast for a 308.

16.5" 300 AAC barrels have 1:7 twist for stabilizing heavy subs; I figure a 308 of the same length & twist will work equally well with subs, while being vastly more capable than the 300 AAC with supers for longer range shooting.

Admittedly, this is probably a solution looking for a problem...
 
To directly answer your question, the price will be the same as if it weren't already chambered...which is to say ~$225-300 depending on which shop you use.
 
Everyone tells me that my 1:8 308 is waaaay to fast too, but I'm guessing they learndeded that on the internets, because it is MOA with several loads from 175 to 240's (sub sonic and full tilt loads). It is harder than you would think to over stabilize a bullet, but you will run in to trouble with too slow of a twist. If I remember right, Robert Silvers made a 1:4 twist .308 to settle this debate. I also used to have a .50 with a 1:12 barrel that shot phenomenally.