Selling in Multiple Currencies
WHMCS lets you sell a product single in multiple currencies. You can set the product price in each currency manually or have WHMCS automatically update it with daily exchange rate fluctuations.
Add a New Currency
To add a new currency, go to Configuration > System Settings > Currencies (Setup > Payments > Currencies prior to WHMCS 8.0).
Use the form at the bottom of the page to enter the new currency's details:

Click Add Currency.
Setting Prices Automatically
WHMCS can automatically calculate the price for products in the new currency based on the existing price in the base currency.
To do this, go to Configuration > System Settings > Currencies (Setup > Payments > Currencies prior to WHMCS 8.0).
Click Update Product Prices.

This example would calculate a price in GBP based on the existing USD price.
Setting Prices Manually
To set fixed prices manually, go to Configuration > System Settings > Products/Services (or, prior to WHMCS 8.0, Setup > Products/Services > Products/Services).
Click the Edit icon for the product:

In the Pricing tab, check Enable for the newly-added currency (in this example, GBP).
In Price, enter the desired price in the new currency.

Click Save Changes.