People give things in exchange for other things depending on the value, why blood?
Blood seems to be the highest priced possession any person could have because the blood is the life of a person. What was Christ paying for that could've cost so much? Sin.
Retribution in a court between two people to settle the score for punitive, collateral,  or emotional  is usually measured in money. 
Why couldn't the price of man's wrongs result in a  type of lifelong monetary installment? 
What is the value of money? 
Money is a bartering medium with an assigned value designed to match the value of the exchanged items. 
People would exchange things of equal value.


An eye for an eye, a tooth for a tooth that is the system of equal value exchange
When we break the law of God it winds up costing the ultimate price why?

Why is forgiveness so expensive?
In fact in the Garden of Eden, the price for disobedience was set from the beginning.
Who set the price for disobedience and unlawfulness? It would seem like God did but why? Is The market value equivalent to his fury?

When you break a law in the United States there are consequences ranging from parking tickets to construction work site double finds all the way to hard prison time. 


I think that blood is assigned the greatest most expensive thing a human being can offer because we are finite beings and we are we exist within the grid of time and our time is limited. God on the other hand exists outside of time and space he knows all things has always existed has all power and has created us all. To have this kind of infinity attribute would come to be an enormous  responsibility. One that is an occasion He easily rises to because he is God. That means that despite what people may say or think about His justice system, unfairness and judgments, He must never break the rules. he must He must be perfect. A concept that's very difficult for we humans to understand.