Newegg offers the refurbished Logitech M570 Wireless Trackball, model no. 910-001799, for $22.99 with free shipping. That's $2 below our November mention and the best deal we've seen for a refurb. (It's also $6 under the lowest total price we could find for a refurb elsewhere.) Features include programmable buttons, USB 2.0 receiver, 2.4GHz wireless, and up to 18 months of battery life.
Note: No warranty information is provided.