$35 for a new mnstr stock is very fair.
however the used price would have to depend on a few things one would be the overall cond IE the labels intact is the can not all beat up and so on.
The size of the comm is also a important factor in figuring a price if the comm has plenty of life or is it to small to be cut again.if you buy a used one that will need a new arm soon for say $15 now add the price of a new arm Generaly a new arm is between $15 to $20 then add brushes ($3 to $5) now you would have been better off just buying a new motor even at full retail whitch is usualy around $40 before buying a used one that doesnt have much life left.
So a good used mnstr with a decent sized comm thats in good cond should go for around $15 tops.
Hope this helps. And by the way say Hi to your dad for me................