It shouldn't be hard to implement if you have a bit of knowledge with PHP. Just use strip mtgox page. I'd use a cron task to save the price to a file to save requesting the page too much. The mtgox source code wouldn't be hard to strip with a simple regular expression finding "<li id="highPrice">High:<span>". You could even implement safety netting to stop it going below a certain cost in case mtgox gets hacked or the value drops for some reason.
Even simpler is using the blockchain API but then you have to trust blockchain. Very simple to pull:
https://blockchain.info/tobtc?currency=USD&value=10These methods could be used to display the cost and if you have even more know how and patience at working out how the shopping cart works you could implement this into the cart.
Shouldn't be too hard. Don't let anyone charge you over the odds to implement it! (although you do want to make sure it's secure if you're giving the script access to your wallet!)