Well, I've played other games that implement a hotkey that is equivalent to 'as much as possible.' Basically when you get an option for "how much: " you can hit > and it will insert the largest # possible. If you have 3000 crystals and are at the deposit screen ">" will insert the value 3000 in the deposit how much field.
I understand, but in Overkill that would be a red herring for new players. As a seasoned player you should never be depositing 100% of your cash on hand in the bank because you should always carry some in case you get into a situation where its life or death.
Having 0 crystals on you is almost certain death when you are fighting a monster and try to surrender.