Though I've yet to actually use mine (never stored the bike this winter), the GEARS ones sold at many bike shops are supposed to be weather resistant, and are seen anywhere from $25-50 depending on size (and if they're on sale.)
My GF is using one for her 250R that's been stored in the garage over the winter, and it seems to work well. Covers the entire bike, even most of the wheels, and has straps to ensure it doesn't blow off in wind etc. There are vents in the top so that any gasoline vapors etc can escape, and I believe it's also to help prevent excessive moisture/condensation buildup.
(edit: I should probably note that this likely means it's not rain/waterproof, of course, due to the vents. But we hadn't purchased ours with rain protection in mind, more to prevent excessive dust, and people sitting on the bikes etc.)