Hi,
By default the culture used is determined by the browser language as discussed on the localization page, it is also possible to force a specific culture as described at the bottom of that page, however, for currency the best way is to make it use the currency specified in the commerce tab in site settings.
You can find code examples of how I do this in WebStore.UI/WebStore/Controls/ProductListControl.ascx/.cs
Hope it helps,
Joe