Newegg offers the refurbished Logitech G502 Proteus Core Tunable Gaming Mouse, model no. 910-004074, for $49.99. Apply coupon code "VISACHECKOUT" and pay with Visa Checkout to cut it to $44.99. With free shipping that's $33 under the lowest total price we could find for a new unit. It features five DPI settings, adjustable weight, center of balance and placement of weights, and 11 programmable buttons. Coupon expires October 31.
A 90-day warranty is included, although it's unclear who backs it.