Actually, "rare" doesn't equal "high demand" in all cases. Obviously, rare games usually are in "high demand", but that is more of a perceived high demand due to the prices that are asked for those games. A game that is high demand is marked by increasing price, regardless of supply, i.e., Faselei! UK complete, with the price nearly doubling in the past year. A game that is rare, but not in high demand is marked, similarly by a declining high $$ value, such as the PC Engine game Sapphire, which has dropped from ~$600 to $300-350 in the past year. Rare, but not exactly high in demand. Since there are so few games in total for the NGPC, most of the games are in demand because the collection is actually obtainable (as opposed to the NES, where you could only hope to get a percentage of the carts, making it not very collectable except for good rare carts and fun/high demand games).
Examples:
Pocket Reversi UK complete = Rare and high demand
Rockman Pocket = not Rare, but in high demand
Sapphire ACD (PC Engine) = very rare, little demand
That ends today's economics lesson. Oh, before I finish, the true measure of value is what you are willing to pay for it. Not what someone wants to sell it for. Consumers generally control cost (look at the price drops for vid systems, computers, etc which are spurred by lack of consumer interest/purchase)