308 is probably the cheapest, easy to find factory load option if you're looking to buy cheap ammo. There isn't a huge difference in cost between any of them once you start looking at the better loads, but 308 is still a little less. And 6.5 CM is usually $1-$2 cheaper for the good stuff than 308. And is just as available.
7-08 is a great choice as a cartridge, but ammo is a lot harder to find and more expensive around here. That may be different in other places.
