My LFS has one with about 3" long. It has two perfectly formed right front legs for a total of five. They say it is not for sale, but what is a good $amnt to offer as we all know money talks!
Never heard of them being common occurance, least not over here. Just sounds like a mutation that would be pretty hard to put a price on. Guess you'd have to think what is it worth to you, because I'm not sure if there would be a going price on them.
I know that's not much help... i dunno 5 times the amount for a regular one? 10 times?