I run one of the maxxis flyweight tubes on the rear at the moment.. It's paper thin and real light, but I have never flatted. They are light to carry as a spare as well. I run 40 to 50 psi in it though. The tubes retail for like $10, but I found them on sale for less than half that. I'm happy with them. You could probably run a tube for a smaller width tire and have the same result though.
Don't know about the $50 tube, but the price tag makes me want one for some reason.
on a side note..
Now that I think about it I have never flatted the rear wheel it's always the front with a thorn or some other small object. Is there any logical reason my front tire flats more?