Check out the Ortlieb Messenger Classic on Amazon: http://www.amazon.com/Ortlieb-Messen...dp/B002DBNOTS/
Waterproof, tough as nails, doesn't have a lot of extra straps and buckles to flap in the wind. I can attest to their ability to keep stuff dry at speed. (Vstrom commuting in Seattle, WA)
AND, at $140, well within your price range.
(Their other bags are great too!)