We often use a currency exchange www.xe.com before purchase, as this gives a good indication of the daily currency rates.
Noting both Paypal and Credit cards will also have a daily currency rate on the date of purchase, which may vary slightly.
At the bottom of our course sales pages, there is a little currency convertor before taking you to the shopping cart. This means you can check the rate at the time of purchase.
It is currently set to US$, although it can be changed to any currency before reaching the shopping cart.