Rarity is a relative term. Production wise, the Mexican Carcass
is incredibly rare as there are probably only a few thousand of them that were ever made. (There were only 10,000 of the American Lunartix produced, so the Mexican figures saw a lower run than that.) But, typically, it's a figure no one cares about. Really, who uses Carcass as anything important in their collection? The only time you see heavy price fluctuations is when there hasn't been one on the market for a while, or when there are a couple of collectors who only need that one figure when it appears for sale.
A year ago, carded Mexican Carcass figures sold for ~$150 each. But, that quickly brought more out of the woodwork and the demand for the figure was quickly sated. There are a lot of these figures out there in the collecting community. Back in the late '90's it was pretty common to find Mexican Lunartix mixed in with American Lunartix at many toy dealers. A shop here in town has had one for sale for years at $40 and no one touched it. It seems that while the production was low, most of the run ended up in the hands of American dealers, so the figures appeared more common than they are.
Personally, I think that $19 is low for a figure of that rarity. This time of year is typically slow for Joe pricing since people have less time and holiday bills. As such, I doubt the next one you see will sell for that price.