Walmart offers the Logitech M570 Wireless Trackball, model no. 910-001799, for $26.65. (Amazon charges the same for Prime members.) Opt for in-store pickup to dodge the $4.97 shipping fee. That's tied with our expired mention from three days ago as the lowest price we've seen for a new unit. (It's also a current low by $9.) Features include programmable buttons, 2.4GHz wireless, and up to 18 months of battery life.