If you'd like to use the easiest script I could find with a stable accurate price index (that won't go to hell just because a single exchange goes down), try adding this to your OP so I can delete (and do not excessively bump to thank me for this post):

$5 = http://btc-priceimg.herokuapp.com/img/5

Just enter this line of code where you want the price to be:

Script courtesy of http://btc-priceimg.herokuapp.com/ - donate to him if you hate me. Or use preev.com if you want to torture your customers with a script that wildly fluctuates spot prices, which is exactly what an average calculator SHOULDN'T do.