This place has USB mice for $0.99 (plus $10 shipping, of course

).
http://www.cableclub.com/default.php?cPath=48It doesn't explicitly say they're ball mice, but they have seperate categories for USB Mouse and USB Optical Mouse, and these are just under USB Mouse. From the tiny photos of the packaging, I don't see anything about optical, and the fact that they're not wheel mice suggests that they're most likely old-style ball mice.
Just thought I'd post, since I know a lot of people are looking for USB ball mice to hack for trackballs/spinners.