The cost of membership is $18 USD a month when you register through the website. For international customers, this is converted and charged to your card at the current exchange rate your card's issuing bank is using. Therefore, the monthly price may be slightly different as a result of changes in the exchange rate. Please contact your bank for their latest exchange rate.
When registering via Apple in the App Store, the cost should appear in the currency set for your account, but will be slightly higher as they apply an additional tax.