Why is iPhone 5 cheaper in US compared to India and UK?

In the US it is only around $649.It is costlier in the UK than in India. In the UK it is around $850.In India it is $827. Why is it so cheaper in the US.

Availability?

Because of shipping fees and tax. The iphone factory in china ships the phones directly to US for checking and quality control. Then Apple US will ship the iphones to Europe and India once checked and approved.

Could also be import taxes. I'm sure there are many factors.