Current 50-500mm $1,059.00
Canon 100-400mm IS L $1,610.00
New 50-500mm OS $1,600.00
Prices for the other two lenses off Amazon.com, 50-500mm OS from above.
Essentially its a very good question, but a quick look at the competition for that lens shows that its already $600 more than its original model and that its current price brings it in line with current pricing on the canon 100-400mm lens. Sigma are often regarded as a cheaper option and part of that is that they do charge less (generally speaking) than canon and nikon on most things (part of what the companies trade on is their name). So Sigma are charging a very fair market rate on this lens. Also as I said above, I belive Sigma tend to price their release price closer to its stable market value rather than canon who tend to overprice quite a bit on the initial retail value
(sorry to always compare to canon, but I don't know nikon or other brands well enough)