Yeah guess I'll put my opinion forward as well, for starters the initial question is one that even the most seasoned collector would struggle to answer. As most collectors know it's difficult to give a going rate on a carded Klaatu let alone a full set of Pali 12 backs. Then there's the question of condition with which the price greatly depends upon, so without a good set of photos you aren't going to get an answer let alone an accurate one.
I know that you "hate" the term what someone is willing to pay, but in this hobby and with anything that is second hand, vintage or antique it can't be compared to walking in to a shop and buying goods with a pre determined manufactures price tag attached. It just doesn't work that way and you are going to have to come to terms with that. So many variables such as rarity, demand, desirability dictate the price at the time it is for sale, you may see a Palitoy Chewy sell for $8000 dollars one week and $3000 the next.
Not too sure who the dealer is that has put this challenge towards you but I find it strange and almost irresponsible to put such a task on the shoulders of someone you are apparently mentoring. This whole thing is bizarre.