I have never found a fully waterproof back back that had the fit and features of more general purpose non-waterproof backpacks. So I ended up getting this clear SeaLine dry bag (REI) to slip inside my backpack. I got the clear one because I  thought I would pull it out of my pack but I find I rarely do that. I use it more like an internal liner or sleeve inside my backpack. I realize this doesn’t answer your question. Apologies there. But I was on a similar hunt and finally gave up for a dry bag and I’m pretty happy with the compromise.

Another benefit of this is I also use it inside my commuter bike bag when pedaling to work in the rain.

Look at packs designed for cavers, such as: http://www.innermountainoutfitters.com/lost-creek-imo-econo-cave-pack/

I agree with robray’s answer… put a drybag inside a backpack. Durable and inexpensive. If that is still too pricey, put a trash compactor bag inside your backpack–worked for me when I was in college.