Well, if the box the worth $500 to you, then get it for 1149. It isn't to me. And good luck selling a Canon 24-105 that you would buy for 1149 for anything close to that amount.
It's not 'the box itself' but rather 'the color of the box'.
I got mine 'white boxed' 24-105 L back in 2008 for around $900 (looks like Canon ships much-much more 6Ds and 5D III kits these days than original 5Ds in early 2008). Probably the lens build quality was also more stable in those days (since I did not hear that many complains about 'unsharp' 24-105 L and 'tested 10 copies = all were bad' then). I used the lens to complement EF-S 10-22 on 400D in my traveling lens setup and always was very happy with the results.
Since now Sigma offers it's lens for around the same $900, I wonder if I would choose this lens over Canon's back in 2008. But... No. I would not: Canon's version is smaller, lighter, it uses normal 77-mm filters and lens caps (same as 10-22).
A Rolex is more expensive than a Breitling. Is the Rolex any better?
An Armani suit is more expensive than a Bertoni suite. Is the Armani suite any better?
A Montblanc fountain pen is more expensive than a Parker fountain pen. It the Montblanc any better?
A Maserati is more expensive than a Porsche. Is the Maserati any better?
the world is full of items we are willing to pay more for, because it gives us some kind of value/quality/prestige/...
A Sigma lens has a reverse engineered EF mount, a long history of AF problems, quality variations pr. copy, numerous examples of poor service etc. etc. To hope for any chunk of the Canon L-lens customer base, they have to be both better and cheaper and they have to be that over time. If they are consistently as good as Canon over time, meaning years, they can move closer to the prices Canon can charge. The only non-Canon brand that can charge as high or even higher prices than Canon is Zeiss. Because they have proved over time that they are consistently delivering absolute top class products in every department. Sigma, Tamron, Tokina and the rest have a loooong way to go before they are in the vicinity of such a position.
I have the Sigma 35/1.4. I am very happy with it, but I am a long way from joining the Sigma fan club