I did a 500 mile tour over mostly unpaved but hard packed trails and the $40.00 Performance Bicycle bag was great. Lots of room, expandable, rigid sides, great pockets and zippers and handy places for attachments. I really like that it attaches with four separate corner Velcro straps rather than two, like so many other bags. Using the straps that the two strap bags have is a pain, particularly if you have fenders. This one goes on and off easily and stays on securely.
TransIt Epic DX Rack Trunk
I have this same bag. Good quality. Well worth the price.