This keeps me fairly dry, and more importantly, warm in a cold rain. It's not critical to stay dry, but if you are at least warm, you can ride for a long time.

http://www.performancebike.com/bikes..._20000__400070

I think I paid about $50 on sale. Also, it comes with a small stuff sack and fits in the pocket of my trunk bag, so it's always with me.