I was just researching this very dilemma...biking bags often have a water resevoir (which I don't need on my 7 mile commute) and seem exceptionally pricey. I went with the Deteur Spider...It has the air channel, but no integrated rain cover. I figured I could buy one separately. I got mine for $63, or about half of what most "biking" backpacks were going for.

Deuter Spider Sack Pack - eBags.com