Use geoplugin.net to automatically show a live conversion of a currency amount on your site.
Usage: [currency]800[/currency] [currency base="USD"]2000[/currency]
Only base="XXX" is supported, where XXX is the three-letter currency code (GBP, USD, EUR etc.) This plugin has GBP as the default base currency. If you want a different default base currency or output format, fork the plugin.
The ~ character is included in the output to indicate that it is only an approximate conversion, as the geoplugin.net rates will be different to the rates you might actually use.