I've recently started using this m-wave-double-day-tripper and really like it. Got it for the cheap price (less than $20) just to see if I would like using panniers vs the sweaty back of a backpack. Not exactly waterproof, but I would say water-resistant (there's a user review that gives it kudos for keeping his stuff dry in rain - it has handled coastal fog for me so far).

http://www.meijer.com/s/m-wave-doubl...bag/_/R-123891

The cheap price almost scared me off, but the reviews (though few) seemed honest and overall said they were worth more.