It should be the amount you paid for your ticket. That amount should have been fixed at the time you made the reservation, saw a price attached to the reservation, and clicked the "pay now" button.

Under the hood, airline ticket fare construction information includes prices in "NUC", a so-called neutral unit of construction which is roughly equal to 1 USD (or 1 Panamanian Balboa).

When you buy your ticket from united.com.jp, you're quoted something like 15000 JPY and charged 15000 JPY but the fare construction will say "this costs 128 NUC". When the charge posts to your credit card, it will be for 15000 JPY; what exchange rate is applied, if any, to your credit card's currency is between you and your CC issuer.