I didn't even think about it until last night, but surely I can get some cheap shower curtains. If I could get two for cheap, that'd be a tarp and a ground cloth. (One each for my wife and I to carry.)
We could probably rig something so that the shower curtain acts as a bivy sack. Or we could make some trash-bag bivy's.
We don't have a large trash can, and I'll have to find some tape, so I'm thinking that might end up being a bit more expensive (over-all, not per use). I'll start comparing prices and see which will work out best for our needs.