Share 'Discs from late 80's and early 90's what are they worth?'

Due to some medical expenses a friend has asked me to see if it would be worth selling some of his discs, such as 1989 classic rocs,1990 san marino rocs and various other plastic. how would I go about finding out how much these are worth to use as a guideline? Disc are new unthrown. Thanks for any help.