The Nikon AF-S Nikkor 105mm f/1.4E ED lens has an 82mm front filter thread as compared to 105mm for the Sigma. I don't know how everyone is coming up with their pricing, but as it relates to front filter thread size... I say it is a bad way to judge price. The Sigma is $1,599 usd and the Nikon runs about $2,500 usd. All the hand wringing over what the final cost will be based on this or that lens is just sillyness. It will be fun revisiting this price arguement once the lens is released... if it ever is.
I get your point, but top quality optics use top quality glass. Top quality glass is very expensive and gets more expensive in large sizes. I am not a technical guess, but at one time had a fetish for nice APO refractors. a 4" refractor may costs $2000 to $3000. An 8" refractor will cost $20,000 or more. A good portion of that difference is the cost of the raw glass,
You can buy cheaper glass of course but most are assuming if Canon were to build a lens as mentioned they will use the top shelf glass. All that being said I would vote for an MSRP around $3500.