I've always liked the idea of, rather having limited editions, increasing the price of a print as it sells more. It seems the best of all worlds - it doesn't artificially limit potential income from a popular print, it allows early purchasers to get a print at a good price, and guarantees to a collector that the print would not decrease (and would probably increase) in value. Limited edition prints seem to be something that galleries favour but I'm not convinced it is in the interests of the photographer.