There is this:
(1.0 BTC)
That calculates an amount based the exchange rate times the quantity from the filename. Specifically, btcticker.appspot.com/mtgox/1.23btc2usd.png would generate an image of 1.23 X [mt gox exchange rate]:
(1.23 BTC)
So if you had a field in a form and some javascript to update that filename on each change in the value of the field, you could get what you want I think.
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http://btcticker.appspot.com/Also, various other approaches here:
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http://en.bitcoin.it/wiki/Converter