Buy.com offers the IOGEAR Multimedia Wireless Keyboard, model no. GKM681R, for $23.99 with free shipping. That's the lowest total price we could find by $13 and the best price we've seen for a wireless keyboard with a trackball without rebate hassles. This 2.4GHz keyboard features an optical trackball, scroll wheel, 16 multimedia hotkeys, and more. It requires two AA batteries (not included).

Note that while this keyboard is Mac compatible, the multimedia hot keys are not supported.