I have a Louis Garneau helmet cover and, the times I've needed it, I've loved it! It's great for cold/windy, too, not just rain. I think you'd eventually get your money's worth out of one.

Check on eBay for warmers- they usually have good deals.

Also, (I know you're broke, but...) Pearl Izumi Calientoes are great for both cold and rain. If it's a total downpour, you'll eventually get soaked, but they keep out the worst of it without going full-blown shoe covers.