Newegg offers the refurbished Logitech M185 Wireless Optical Mouse in Gray, model no. 910-003922, for $6.99. Coupon code "EMCPCWB43" cuts it to $5.24. With 99 cents for shipping, that's the lowest total price we could find for a refurb by $2, although we saw it for a buck less three weeks ago. This 2.4GHz portable mouse features a nano receiver and requires a AA battery.
Of note, a 90-day warranty applies, although it is unclear who backs it