http://www.thinktankphoto.com/categories/rain-covers/hydrophobias.aspx
Not exactly inexpensive but you get what you pay for...
I recently tried an 'inexpensive' solution (a $10, see-through plastic slip-on 'condom' type) which was too frustrating. While easily foldable into your pocket / poncho, these types don't repel the water very well and your hands get more wet trying to get your hands into them.