I have had good luck with the no-name spicer cog. I think it is less than $10, I got it when I was jobless and haven't found a need to replace it now that I have a job...

http://spicercycles.com/index.cgi?ca...cat_desc=Track. Yep, $7.

Fast/cheap shipping, awesome customer service.

[edit]has anyone tried these and had a bad experience?[/edit]