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Racketboy has a great article where he picks out some of the rarest and most expensive games that collectors seek out. The list has twenty rarities including Panzer Dragoon Saga (coming in at #20), Metal Slug for Neo-Geo (#10) and Air Raid for the Atari 2600 (#7). Believe it or not the top two games fetch for over ten grand. Some people really do a PAL version of Kizuna Encounter for Neo-Geo instead of the USA or Japanese AES, which cost around $50. The price of the PAL version somewhere between $12,000-$13,500.
