How he earned the money is irrelevant (as to damages). He would only be able to get the amount of damages he could prove as the value at the time. What are used bongs going for these days?
By the way, unless you broke the bong accidently while opening drawers in his room while looking for your Bible so you could research the concept of Agape for your lecture to your Sunday school class next week and you only found out about the smoking, selling and other possession when your friend spilled his guts in anger at the loss of his expensive drug ingestion device, you should slow down with thinking you are home free. To prevail at the end of the day, you would testify to these facts in court.
It is rarely a good idea to admit to committing crimes under oath. (Yes, you. You've either been smoking or had possession or conspired to sell the pot yourself to know the things you're claiming.)