Today is the last day of the eBay 20% off select tech products/stores and participating eBay seller RedTagCamera has a few of the popular Sigma 17-50mm f/2.8 lenses available in both Nikon and Canon flavors.
At the time of this posting, the seller only had 5 Canon, and 6 Nikon ones left.
Canon: http://www.ebay.com/itm/222417193314
Nikon: http://www.ebay.com/itm/222375973663
Apply eBay promo code PSHOPTECH417 at checkout to drop price to $250. This is a pretty slick price for this very popular and versatile lens.