Take a look at Fu'un Super Tag Battle, better known as Kizuna Encounter as the perfect example of how a mediocre game can create synthetic demand to raise prices sky high on the weight of a limited production run. So are we to understand that buying a lame game that will naturally have low production naturally be a sound investment? - in a word.. No!
I have been studying the market for many years and have to agree with other collectors that the internet has definately ruined the hobby for the rest of us. Problem is that you get gamers who decide to become collectors over night read up on an apparently sought after game, jump on eBay and stop at nothing to secure a copy. Final Fight Revenge for the Saturn is on record one of the worst 3D fighters you could possibly play, yet the self proclaimed gaming cognoscenti identified it as a collectable gem as it was the last game for the system, incidentally produced by Capcom. Behold, prices soared to triple what was being initially asked on the weight of a few unscrupulous types screaming "fire" and the rest of us getting burnt. An example of rare, collectable and sound investments come in the shape of programming marvels like Metal Slug AES, Radiant Silvergun on Saturn and Border Down on Dreamcast... Rendering Ranger R2 for the Super Famicom is worth a mention too.
The major greviance among collectors seems to be with AES carts that are essentially all rare if you consider the home system was a niche market product. Take a look at Fatal Fury Special and compare it to the aforementioned Kizuna Encounter and the price difference is substancial. KE's alleged super limited european release last sold for somewhere around $12.000, this has ensured that the price of the Japanese version has shot up in line although most would agree that Fatal Fury Special is a better game.
Once again the internet and some secret wiseman commitee seem to decide these prices by creating panic among collectors, hording stock and bumping up the asking price!
Best advice is to hold on paying out like a broken fruit machine for a particular game unless it's one of those gaming grails that is openly hailed by the elite.