Because a 10mm focal length lens is different from an 18mm focal length lens. Aperture, while important, is simply how much light the lens can let in (it affects depth of field as well); it has nothing to do with the angle and field of view of the lens. This lens and your lens should look very similar when they are both zoomed to the same focal lengths (18-24mm, the only focal lengths where they overlap). If you buy this lens and only use it from 18-24mm you are wasting your money. The reason people buy this lens for its extremely wide focal lengths (10-14mm). They give a perspective that cannot be gained in any other way, especially on DX. Few lenses get this wide and even less get this wide on DX. This is the only lens from Nikon that gets this wide on DX. That's why it's so expensive. Despite what someone else said, this is not a pro lens. I'm not sure any of the DX lenses are pro lenses. It is very much consumer grade quality; it has cheaper construction and more distortion. The Nikon 12-24mm is semi-pro (better, more durable construction) and is accordingly more expensive. But this one has a wider angle of view (shorter focal length), and that's what will be key to most folks. Just be careful with it and the distortion can be corrected with software.
As of 5 November 2013 the price in various places (Amazon, Adorama, B&H Photo) is $810. I don't know if Amazon will match prices, but most of the reputable places' prices mimic each other and, for the most part, have comparable return policies. If you find it cheaper from a place you trust with a comparable return policy, buy it there. There is no reason to be loyal to Amazon if they can't/won't match it. It will only cost you money unnecessarily.
Hi Karen,I have used this lens on a D90,a D7000,and now on my D7100.It works great and is a super lens.It will no doubt cost more than the Tokina,but if you have the money go for the Nikon,I am sure you will not regret it.Enjoy anyway,whatever you buy.