This money could go to a bank account. We could buy temporary buffs for certain categories of characters with it (gifts ? Flowers for a princess, jewels for dragons, etc. That would be like the runes, but with minor and temporary bonus)= it would be re-injected in the next runs.
I know it must be difficult for the game mechanism to have another market place ( we can already buy things with diamonds and challenge/dungeons coins), however the fact that this money is disappearing is not "logic" story wise.
Thank you for expanding. I'll note it down and we'll see what happens.