There is a guy in wisconsin who has a functional 37mm US anti tank gun. on wheels and with ammo, I think the last time I saw one on the Market if was in excess of 75,000. Because no HE ammo was ever made for the Gun, I do believe it is the only one of its type that is possible to buy.
many years ago I was in guatemala on a missions trip and we were able to walk in to town to see what was going on, In town was a gun shop that had a M2 for a couple thousand, g-3's and ak for about a hundred. US property marked M-16 in A1 configurations for about 75, demand was for the AK as it worked better in the jungle. Every adult male there carried something. M1 carbines, M1's m3 greaseguns mausers, FAL's were there too. mines grenades you name it. it was all stuff that came across the border during the contra, drug wars. Out front was one of those russian AA mounts with a 14.7 (iirc) dual purpose. quite fun.