Also to note, is that if you trade currencies, you have to pay for it - so even if the trade is like 1:2 or whatever, you wouldn't just get double the amount becuase you pay 'x' percent to whoever converts for you.

I agree the rates would be very volatile - like during depressions and stuff currency value plummets, and that would work between wizard and Muggle currency or between different types of wizard currency (like country to country). Also, currency exchanges are based on a lot of complex things, like economic expectation and country's GDP....so it's actually fairly complex to come up with the rates, I think.