we all know cards such as thanatos can go up to 100b, samurai 50b, meg is 40b and so on. but, do you know how the zeny introduced in the first place? how can those bilions of zeny flying around, and where are they come from?
well, the only way (afaik) zeny is introduced to server is by selling something to NPC and that NPC will give you zeny which will be circulated thru the server. the amount is getting bigger if people keep selling something to NPC.
now, which kind of player contribute most of zeny introduction on server? prankly saying, i think they are bots. those mindless drones who hunt trash items all day and sell them to NPC which generated zeny to the server. how about normal player? sure there is many people like to hunt in magma dungeon, or greeding in TI, and sell those junks to NPC for zeny. but i believe the contribution of bots on generating zeny in the server is greater than normal player.
well, it is the black truth i hate to admit. And also i dont actually say that whether having a lot of zeny in server is good thing or bad thing, but i can say those mindless drones are the 'hero' of this phenomenon. by introducing more zeny in server, the server will be inflated when all things will increase in price (but NPC items still have same price), and one way to counter it is by introducing zeny sink like raffler event or such.
TL;DR: BOTS ARE THE ONE WHO MAKE THINGS HAVE PRICE OF SKYSCRAPPER