What's it worth? 70's aluminum Yenko block, never used
 

I work with a guy who has an aluminum Yenko block, I am guessing one of the 70's blocks Yenko had cast. It has never been used, his father purchased it decades ago. I thought it would make a really cool addition to my 1968 L78 Camaro. Anyway, what's a realistic value on this block? I don't think my pockets are deep enough to make it fair to him, but I have to ask. Thanks, Tom