It's a good lens and was probably discontinued after running alongside the same company's 150mm F2.8 macro for some time (and especially after they upgraded that lens to an OS type). This company still seems to have it in stock:
http://www.microglobe.co.uk/sigma-18...ns-p-1900.html and I'm not surprised at that price! It was heavily discounted, I believe, to get rid of old stock so they'll not attract too much custom at their current price.
Whatever, it's a long way short of being categorised as 'antique'