THe Iphone users have the highest demo (they spend by far the most per phone in use), so it is highly probably that that people with Iphone control more than half of US money spent on transactions.
Still, I said in some other post, it is unlikely that they'd get more than 20% of all US money spent on transactions even in 5 years. That's still a bit more than 1B. More than the cost of software that's for sure ;-).
multiply by the whole world.. there's tons of money to be made by apple with applepay.. #loads.. not sure why you're trying to downplay it ?