I have some experience of attempting to make an Instant Airport Upgrade, with attempt being the key word. It used to cost the same number of miles as an upgrade in advance, but now the miles cost is astronomical, even with the 50% Platinum discount (my partner is Platinum). I last attempted this in October 2018, and the one-way upgrade cost AUH-MAD was more than a round-trip GuestSeat redemption. I would strongly advise you to use cash or miles in advance as the airport upgrades are no longer the deal they used to be.