I've done pretty well on Ebay. Just watch the shipping. I've bought full cases, just entrees, just accessory packs and grab-bags. Some offered flat rate shipping in priority mail boxes and I got great shipping from a guy who was selling by the case. Just look around and bypass the guys selling entrees for $4 each with $2 shipping.
I have not invested in MRE's, looked at some local sports stores and found them really lacking in calories. Some are in the 300-400 range. A person would have to eat 5-6 of these minimum just to get along. Long Life Food Depot does say their full meals are in the 1250 calories range. Just gotta be careful in what one buys. By the way, OP are you really looking for "cheapest quality" or the cheapest, best quality?