It's hard to tell why some things go for exorbitant prices and other things that are rare never sell. Of course it takes two stubborn people to run the bidding up, and who knows why each of them wants the thing. New collectors tend to pay a lot for crap when they first start collecting, as they are unfamiliar with how available things are and how much they should go for. Advanced collectors might only need that one item to complete their collection, so they throw caution to the wind and run the bidding up. Sometimes, they might only want one thing in their collection, and the thing they are bidding on might be something they fired or handled when they were in the military, so they throw caution to the wind and drive up the prices. The dangerous ones are the ones with unlimited cash, and an agenda. They want it for their personal collection to try to impress their buddies, or they know how they can resell it to make a lot more money. A company was mentioned earlier in this thread, that makes repros. They have purchased Russian projectiles from collections that were sold on Gunbroker, so they could have a examples to copy, so they could sell inert resin/foam copies to the military for hundreds of dollars per copy. They get their investment back on the first sale. This type of purchase is particularly infuriating, as the item has been removed from a private collection and the market is flooded with counterfeits that are too expensive for the common collector to buy and available for sale only to the military.
Another thing that has driven prices skyward, was the 9/11 attack and all the new security procedures that keep us from having any ordnance shipped into the U.S. Things that used to be scrapped out are now completely demilled and run through chipping machines that turn the metal into chips the size of poker chips. Our government is totally paranoid that someone might somehow remilitarize projectiles, cartridge cases, etc., so they destroy everything, thus making the items in collections more rare and collectable, and thus more expensive.