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Why do financial institutions charge so much to convert currency?
As mentioned in several other answers, the main reason for high rates is to maximize profit. However, here is another, smaller effect: The typical flow of getting money from an ATM: Suppose you have a minute to consider the offer, then in that time the currency may drop or rise (which you can see from an external source of information). Therefore this opens a window for abuse. For real major currencies these huge switches are rare, but they do happen. And when 1 or 2 minor currencies are involved these switches are more common. Just looking at a random pair for today (Botswana Pula to Haitian Gourde) I immediately spotted a moment where the exchange rate jumped by more than 2 %. This may not be the best example, but it shows why a large margin is desirable. Note that this argument only holds for when the customer knows in advance what the exchange rate would be, for cases where it is calculated afterwards I have not found any valid excuse for such large margins (except that it allows them to offer other services at a lower price because these transaction).