There are, but not many for free. If you wanted to do it for free, you could include a link saying something like "To see an estimate of what this will cost in your own currency, click here" and link to a Google search for "xxxUSD in GBP". There's some other sites you can do that with as well.
(While writing this, I stumbled across http://www.webservicex.net/ws/WSDetails.aspx?CATID=2&WSID=10 - it appears to be free, or rather there's no way to pay them, but requires you have SOAP on your server. Perhaps cache the rates once a day and use them on the site?)