Custom Credit Tracking

I'd like the ability to manually add a credit track to a board, and then when a card is set, add a value to the card, and it deducts that value from the boards set balance. 

Case in point, I have a client who purchased 64 credits, and each credit is worth $450. When he requests a new task, the scope of work determines how many credits. Because forms have no logic to handle this, it would be great to provide a comment on the card after it's created and eventually, show him what his current balance is. It would be great if the function had the ability to name the unit, the value per unit, and how many units there is. 

Bonus if you could make it track a ledge of when new credit units are added or deducted.