I've been watching these waterproof backpacks in Costco. They're currently clearanced at $14.97:
http://www.over-board.co.uk/catalog/...itres.p79.html

Basically, similar to a drybag with straps on the back - similar to ortlieb's stuff as well.

I bought 2 of them, and am planning on making poor woman's waterproof panniers out of them. The webbing at the top of the backpack harness will be easy enough to modify into strapping to a rack - and the waist part at the bottom to strap the bottom to the rack. I'll probably leave the backpack straps on to use to carry them when I take them off the rack - but if you wanted to save weight you could cut it off (these come out about 2 lbs heavier than the ortlieb back rollers, a little bigger and like $120 cheaper)

Anyways - just in case one of you wanted to try your hand at pannier making or would just like a waterproof backpack for commuting, these look pretty promising to me.