Ebay/Browser App

I am wanting to build an application that when a user is browsing Ebay for gold or silver bullion, the application (sitting permanently in front of the browser) reads the mass of gold or silver - in grams ('g)' or ounces ('oz') in the item description and converts it to a price in whatever currency the user chooses. It would be good if the application could work with all different web browsers. Also I would look at doing a Mac version also if you can do this. It would also be good if the application could automatically get gold and silver prices (and currency conversion rates) from the internet. This is not essential, but if you can do this then I will look favourably on bids that can do this. I've temporarily called the application 'LIvePrice' The final product deliverable is to be an installer file that requires the installer to agree to terms and conditions before it will install. No copy protection is required.