I've actually joked around with this idea from folks, but I'll add it here. Basically what my Idea is a small vending machine that runs on silver coins and gold coins. It can be twice a day but the creature it gives you is completely random. You however have the option to reject what was offered to collect "points". The higher the amount of points you have, the higher chance you have for the creature obtained via vending machine to be rarer than usual. Points attained also may have higher values depending on the rarity of what was given back. Points are consumed when you finally grab one but it is in varying degrees depending on how rare the creature you obtained was. When using gold coins you get a multiplier bonus for the points you have plus a higher chance to gain a rare creature, but but all points would be consumed if you grab the creature.

 

There is also a machine next to the vendor next to the machine that exchanges the "paper money" for its coin derivative.

I've seen this in other games, it could be good, but it's usually done in such a way you feel like standing at a gambler's desk.