Newegg offers the refurbished Logitech M570 Wireless Trackball, model no. 910-001799, for $19.99 with free shipping. That's $3 under our refurb mention from last winter and the lowest total price we've seen for this trackball. (It's currently $7 below the best price for a new one.) Features include programmable buttons, 2.4GHz wireless, and up to 18 months of battery life.
A 90-day warranty applies, but it's unclear who backs it.