I think rarity is the main factor, but popularity also plays a role. When people start searching for a skin en masse, its price immediately increases. I bought Desert Eagle Kumicho Dragon on
https://dmarket.com/csgo-skins/produ...kumicho-dragon , and I can say that it will cost more than the usual Desert Eagle, because it is unique and attracts the attention of players. Appearance also plays a role, but if the skin is something epic, like the same Kumicho Dragon, then the price soars, even if it is not that popular.