It depends whether it's me personally who's running out of coupons or the entire supply of coupons is being reduced. If my ability to get the product for the same price is diminished then the price is being effectively raised.

In this case I'd agree that pricing is effectively raised as 10$ > 10$ - 50%, there's no need to complicate it. However this is not even the right metric for this problem, a better one would be total spent / work produced. If all customers spend more money for the same amount of work (adjusted to progress) then clearly the price is increasing. This would be true in this example as well.