Noon Solar Bags' going price at the moment is around $450 for a bag. They look fantastic mixing old craft processes with new age ideas.
Eclipse Solar Gear makes the Fusion messenger bag sold for a much cooler $190. The style is urban but the panel looks small.
Juice Bags by Esc Outdoors go for $200 and look rugged enough for adventure. Built with a padded laptop sleeve and flexible ballistic nylon solar panels instead of solid panels, this could be a worthwhile solution.
Or, for the DIY-er in all of us, one could buy a flexible 12 volt solar panel and sew it into an existing bag. PowerFilm Inc. manufactures super thin and lightweight solar panels. Silicone Solar Inc. has a diverse line of consumer ready flexible solar panels, and they are waterproof. DIY Instructable on building a solar bag.