Didn't read through the whole thread, but did anyone suggest Ortliebs already?

Seriously, if you distribute the purchasing cost over the life span, it's not that expensive. My back roller Classics are maybe 10 years old now. They are used in all kinds of weather, including really cold spells. When I go paddling, I use them as dry sacks in the kayak, so they get exposed to sea water too. They look very used, but are still 100% waterproof. The only thing I've had to fix so far is the plastic locking part of the closing strap that runs over the top. Had one of them snap in the winter (very cold), the closest Ortlieb distributor promptly provided me with two spares at a reasonable cost, and suggested I change the other one too as they have improved their design to be more durable. This was several years ago.

--J