like he said about .308 need a different lower, but the 6.8 spc uppers will work with a standard lower. 6.8 is fine for deer.
on the pricing subject. it all depends on what you want. if someone offers an upper with exactly the stuff you want, it may be about break even. but if you buy a complete upper, then start switching out parts. you are spending money you didn't need too.
i am currently in the process of building a coyote gun. i have everything but the barrel. the barrel is on order from krieger and should be here sometime in january. i started this particular build in January of this year.
so 1 year assembly date, but the sticker shock didn't heit me all at once because i spread it out over the entire year. total on my rifle will be approx 2300. now i could've saved some money by buying a varmint upper from bushmaster or what not. but i got exactly the parts i wanted.
so first figure out what you want, then start pricing it all. assembly is not hard