There are a few factors that affect the high cost of those packs.
1) It's LBT, they don't give a shit about onesy twoseys from individual dudes, mil dudes buy in bulk and don't ask what it costs.
2) The costs of nylon goods have gone up all around, nylon is a petrolium based material.
3) Good backpacks take a long time to make, every time I sit down to do one, I forget that it is almost double the time of making an armor carrier. Good packs around the $150 mark take much more time than a vest I can sell for $200+.