Tax on "unbundled" price of iPhone?

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yes, I think I heard that before. the difference between unlocked and contract phone is $450, over two years that comes to $18.75/month. Some states claim that you should pay all tax up front. Then I wonder if each month you pay sales tax again on the entire bill (including the $18.75 which goes to the phone) or only on the remaining portion of the monthly bill.