I use Seal Line dry bags and they work well to keep my sleeping bag and pad dry. I have the baja and see bags, which aren't what you are looking for. I checked their website, www.seallinegear.com, and they have a whole line of bags, including duffle bags that fold and zip. I don't know if their prices are in line with what you want to spend, but that should give you an idea of what another manufacurer has to offer.

For the stuff in my panniers, I use two-gallon ziplock bags.

Hope that helps.