It's not that the LMT and CMMG uppers are so high, it's that the Stag uppers are so low.
I haven't noticed much of a difference in prices for complete uppers between CMMG and RRA and Bushmaster.
Stags are about 10% lower.
LMT's on the other hand are about 10% more. I'm not sure about their uppers, but I have an LMT lower and the nicer parts in it and the telestocks go a long way towards explaining why they have a $330 price tag instead of around $265 like several other makers.
Anything Colt costs and arm and a leg, regardless of whether or not it is worth it (it's worth it in some cases, but if you could afford it you probably wouldn't have posted this thread).
Price differences happen for all sorts of reasons, many of which don't have anything directly to do with the quality of the product. Things like how big their operation is, shifting demand from us AR nuts, etc. Manufacturers also sometimes cut their prices to get their foot in the door.
If you're looking at Stag, LMT, and CMMG uppers, you're probably looking for an M4 type carbine upper. If the cost difference between an LMT upper and an RRA or Stag upper is enough to make a big difference to you, then by all means get a Stag upper or a BCM upper from Bravo Co.