Thank you for the pictures and showing your method. I have the same experience about copper volume as miked said. In a heavier ones is less percentage of copper per weight than in the small ones. It's getting to 7% in some heaviers and the most I got was from 24lbs one, similar as you posted (14%). When we know this, it's easy to calculate the profits and what should we pay for them. In my country (Czech), usual price per kg of unstripped motor is $0.40. I would never pay so much as you, it's not worth the time, as you perfectly documented.