I don't wanna ask yet another question but what makes surplus weapons cheaper? Is it because they just have newer items so the old stock becomes something they want to get rid of?
You got it! Sunk cost in something that really isn't needed for anything anymore.
The Soviet Union set up their factories to make things like Mosin Nagant rifles, SKS carbines, and AK-47s/AKMs/AK-74s, (along with heavier stuff) and simply go, go, GO. They made millions and millions, and sent them to every corner of their union, and to all their outlying puppet states -- and then sent the technology, including whole factories to those places as well, and told them go, go, GO!
The eastern bloc was very long on mass production and rugged reliability (with weapons at least) but very short on efficiency and inventory control.
If you get the chance, read C.J. Chivers'
The Gun. Tons of great info in there about this stuff. I don't have the book right to hand at the moment, but he cites one of the ex-eastern bloc countries (Bulgaria I think. Maybe it was Romania.) as conducting an inventory during the last decade to try and figure out how much of their military gear was still sitting around and hadn't been stolen and/or sold on the black market. At the time, even with all the losses, they had on hand
one hundred AKMs for every single soldier in their army.
Wow.
And that's just one type of weapon owned by one small satellite state of an enormous empire which produced enormous quantities of a bunch of different weapons. They literally filled up bunkers with cases of rifles, then closed the door and started filling up the next bunker, and so on.
The Soviets were the kings of this, of course, but the world is also full of Enfields, Mausers (of many makes), older surplussed US M1s, M1903s, M1917s, M1 Carbines, M14s (not so many...short service life and no one else really wanted them), Carcanos, French Mas rifles, Swiss K-31s, and newer stuff, too. All left over from the last war, or the one before that, or the one before that, or the ones folks expected but which never came.
Since there are "enough" military rifles in the world for every single soldier the world over to have more than he could possibly carry, the demand becomes low for anything that isn't the latest and the greatest. When the prime buyers don't want something, anyone who might can pick it up pretty cheap.