Sorry guys, but if you're asking the question, you really don't understand the mechanics.......
The other profit barrels aren't heads paced until you fit them to the action, put in a go and no go gauge and set the barrel nut. They are NOT headspaced by the barrel manufacturer and therefore not much time/expertise is taken up in the machining (well not as much anyway).
AI barrels are headspaced from the factory so you just screw them into the action until they stop, then do up the cap screws (to 49 inch pounds, or as we say here in Aus "bloody tight") and you're good to go. There is a LOT more care required in this, so a lot more $.
Oh, and yeah, if you can afford an AI, then apparently you can afford a barrel. However I can't bring myself to buy a .308 barrel because it would cost me more than my Kimber Tactical in .308 did and it shoots under 1/2 MOA.....