This works for both Canon or Nikon mount!
Abe's of Maine currently has a December Offer: 8% Off Selected Lenses!
You must add to cart to see the price of $459. Then add a promotion code "loyalty15" for another $15 off, which brings the price down to $444 shipped.
If you want to save another $20, then sign up Bill Me Later (for new customers).
This probably of the lowest price you'll find for a brand new Sigma 50.
Also, you can get the Sigma 30 f/1.4 for the same price if you missed out the Thanksgiving sale!