Yahoo! Auctions Japan, home of everything a Rockman collector could ever want, is hosting two very interesting auctions that might interest you. A lone individual is auctioning off the insanely rare Rockman 4 and Rockman 5 gold cartridges—items issued as prizes for winning Robot Master design contests back in the day.
Only eight copies of the Rockman 4 cart exist, two of which we know their exact location: one resides in the company of Japanese game store Super Potato (retailing for $6,000), while the other is said to be in the hands of Bright Man's (RM4) designer. As for the Rockman 5 cartridge, I personally had no idea a gold cart version even existed, so I guess we can assume eight of those exist as well.
With a starting price of 1,000 Yen each ($11.00) and zero bids, I highly recommend dropping the money on either of the two, but please do take into consideration that these may or may not be legitimate. The starting price alone makes me feel very uncomfortable, almost as if these are intended fakes. Still, there is a strong possibility these are legit and the seller has no real understanding of how much these are worth. I don't know, It's your call.