The tax isn't the issue. It's the fact that I.e. you're paying tax on $778 when you're only paying $389 in payments. So every year you upgrade you're paying tax on $778 even though you're only making $389 in actual payments. That alone makes the program more expensive than anticipated.
Every year you upgrade, whether you are paying full price or financing it through Apple's Upgrade Program, or your own Credit Card, or your little sister's piggy bank, you will be paying the full tax. Let's do some math:
Let's make it simple and say tax is 10% or, $78
Apple Upgrade Plan
$32.41/24months (
iPhone6S 16GB+AppleCare) + $78
Sales Tax =
$855.84
Pay in Full
$649
iPhone6S 16GB + $129
AppleCare + $78
Sales Tax =
$856
So in two years, it'll be the same if you decide not to upgrade after the first year.
Now, lets see how it goes if you decide to upgrade in a year.
At the anniversay, you would have spent:
Apple Upgrade Plan
$388.92 (12 months of payments) + $78
Sales Tax =
$466.92
Pay in Full
$649 + $129
AppleCare + $78
Sales Tax =
$856
This means you would have to sell the
iPhone6S 16GB for at least $389.08 ($856-$466.92) to break even with upgrade plan.
Granted, if you sold it for more, you will be better off. If you sell it for less (bad wear and tear), you'll be worse off.
In order to get the iPhone 7:
At the end of second year.
Apple Upgrade Plan (you simply trade in your
iPhone6S 16GB)
$32.41/month/12months (
iPhone7 16GB+
AppleCare) + $78 Sales Tax =
$466.92
Pay in Full
$649 iPhone7 16GB + $129 AppleCare + $78 Sales Tax = $856 - $389.08(
if you sold iPhone6S 16GB at this price) =
$466.92
So it boils down to this.
IF you can sell the iPhone 6S 16GB for at least $389.08 after 1 year, you would have spent the same exact amount of money.
Now, you must take into consideration the wear and tear, which easily reduces your sale price. For the trade in program, reasonable wear and tear does not affect the price (trade in value) since Apple will take it in and give you an upgrade. This scenario also assumes the price of the iPhone 7 will be the same price at launch day as the iPhone 6S at launch day.