Zozi offers the Timbuk2 Yield Pannier Bag in Black (pictured) for $35 plus $7 for shipping. That's the lowest total price we could find by $47. It measures about 11" x 18" x 6" and features an internal stretch pocket that fits up to a 15" MacBook, reflective accents for low-light visibility, and a vista loop for attaching bike lights.
Also available is the Timbuk2 Cog Pannier for $55 plus $7 for shipping. That's the best price we could find by $38. This bag clips directly to your bike as a pannier attachment, but can also be quickly converted to a shoulder bag with the removable, padded strap. It features an oversized top flap, fully-padded laptop sleeve, and external zip compartment.