It is no longer available but was worth 5 passes in the Black Market. It's worth now is relative to how much a player is willing to pay for a horse with the element attached, since the item itself is no longer available by itself.
9/10/11: The 5th Element is not available through the Black Market but through special offers from Howrse some players have them and you can trade for them through the Item Exchange. Actually, they are having halloween cards until tomarrow on 11-11-12 and if you find the certain amount of so and so cards you have a chance of winning the 5th element ;)
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You can buy another fifth element and change them but there is no guarantee you will get a different element type.