OK, as my friend Skypainter tells me, this rapidly becomes an obsession.
I've priced out a bunch of things, but what say you to this?
I own a pretty big mosquito net - I imagine it won't be so hard to adapt it to cover a hammock. (Free!)
Then one of the Grand Trunk singles for around $50
And a cheap tarp from Campmor (like $5-$10, I know there's a weight penalty for the polyetheline, but I'm not an ounce-counter.)
That would do for the summer, then I'd worry about figuring out how to winterize it later.