What you were expecting to happen, and what actually happened:
I clicked across until it said enough for Mythic. I expected to have enough of them to upgrade to Mythic.
What happened, it bought enough at 1 copy per pack. As it is rare, it is 2 copies per. Therefore I ended up paying double the amount than I needed to, to get it to Mythic, and ended up with a lot more copies than needed.
Saw this too on iOS. I thought it sounded too expensive, so I only used the shortcut to buy to legendary. Definitely looks like the computation doesn’t account for getting 2x per purchase.