I think you need to have 3 prices for each game - High, Low, and Average. Also taking in consideration box and manual, and complete. Also price shoud be shipped price paid (or estimated shipping).

Ex: (Low, Avg, & High for each of these)

Game only
Game w/ box
Game w/ instructions
Game complete

Earthbound - 23 35 60
Earthbound w/ Box - 40 50 70
Earthbound w/ Guide - 45 55 75
Earthbound Complete - 60 80 150

Chrono Trigger - 32 35 45
Chrono Trigger w/ box - 35 45 50
Chrono Trigger w/ instructions - 35 45 50
Chrono Trigger Complete - 45 60 90

Also should take in consideration the condition of the items. I can do estimates on SNES games real quick, because I have been very familiar with it, but out of the loop for about 6 months now.