Creature pricing criterias:
I) Rarity. (that can't be recruited normally, or those deeper in the MDshop, or other specials like Morphs, Tainted Angiens, Colorless Joker, etc). If you can recruite it normally from places with easy acces, then it's not a rare creature.
II) Age, Tokens, Stored heat. (If the creature is rare, these criterias will boost the price for sure. And even if it's not rare, it can cost allot if it has allot of Heat, Age and fitting Tokens)
Also, most "green" items (how I like to call them) like shared tools [example: axes, buckets, dices] can be taken by anyone from a specific public location and they worth nothing in Coins (maybe exept the Heat Jar and Reality Coagulator), so don't get fooled if anyone asks you for coins/creatures/other items in exchange for shared tools.
Resources are also a source of income (a farely weak one at the moment), They're price depending on how hard it is to gather them, and on how many they are..
But if you want to be a resource gatherer, you should first find people who need, then gather how much they need, because most of the times.. the more resources you have the harder they are to sell, because you can't split them. (Example: If you trade branches to someone, you will trade ALL of them no matter how many you have, with no way to split them in smaller stacks)