As to Nork, you are a cool member over at DP but I have to disagree with you. I just sold a complete copy of electroplankton for $90 on amazon less than a week ago. Things sell for crazy amounts on amazon. ESPECIALLY strategy guides. This is NOT to say people should use amazon as a judge of worth, though. If it is up that high for a game, chances are it will sell if there are several up and it has at least some demand and they are all priced similarly. Amazon is crazy and makes no sense (lot of items way too high, but just as many too low).
As for electroplankton, it pulls about $50-$60 complete on ebay in MINT condition. Sealed it has been pulling $100 or so recently. Higher than it should pull, but it is getting harder to find so it does make some sense.