$89 on Amazon.
I kinda like this idea - wash your hands, and the water from washing your hands helps flush the toilet later. (I think as you wash after you flush, it helps fill the tank rather then just using fresh city water.) Won't work with a lot of bus toilets though. . .
What's your goal with the fold-up sink - more countertop, or just a way to wash your hands in the bathroom? If you could install it on the wall over the toilet so it folded down over the toilet bowl, you could get dual functionality (just not at the same time.)
This might be a little silly, but what about hard-mounting a faucet to the wall, but making the sink out of plastic of some sort with a metal frame? Think of it like washing your hands in a giant ziploc bag - you un-zip it from the wall, and you've got a nice, wide "mouth" to catch the water, but "zip" it closed again, and it folds up flat. (I'm thinking about some of the people who've made showers from a large metal tub and a shower curtain - the solid part is the plumbing, the rest of the space is just a plastic curtain.)