This is not something where the code can simply be written on a post. You'll need to make a front-end which displays data processed from api's like Google Finance.
Try checking out the api documentation at openexchangerates. It's all easy stuff if you understand the API and know how to decode data delivery types like JSON using PHP or Javascript/JQuery.
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In "Post a PHP snippet" I posted a fully functional PHP function to convert currencies in real time using Yahoo Finance. It's also good for stocks. The quotes from Yahoo are very accurate.