FWIW if I were trying to compare the prices evenly I would use Excel's PERCENTRANK function. Of course you'd want a large number (100+) of prices for each commodity/equity, for this method. As others have said, it's easier to compare the percent changes of the prices, and/or use CORREL. However with PERCENTRANK you would be able to draw some graphs and quickly visually tell if further investigation was warranted...
I use this info in my analysis and portfolio management: http://understandmarket.com/index.php?categoryid=55
a portfolio's correlation is obtained by taking the correlation between each pair in the portfolio and then averaging them. you can do it in a few seconds or pay someone a lot to do it for you.