eBay (via Deals-All-Year) has the Sigma 17-50mm f/2.8 EX DC OS HSM Lens for Canon available for $ 304.99 with free shipping. Compare at $ 469.00.
A Nikon version is available for the same price via this auction.
Note: These are likely grey market items and therefore ineligible for Sigma USA warranty (and possibly service).
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