Try this site:

http://sites.google.com/site/rtwfares/home

Go to the bottom of the page, click the last active link: "Download". Then go to the bottom of the office document that opens, the last line has "oneworld" as one of the options. Click. Then you can play with fare class, number of continents, starting points. Not always right up to date, and doesn't include taxes and fees, but relative value should be close. Then take your most likely options over to the official Oneworld site and see the current fare (you'll need to open a currency converter, which is why I like Sagy's spreadsheet.)

Also great info in the stickies, and most of it in the first posts!