Office Max/Office Depot/Staples almost always have reasonably priced laptop bags in stock, including really well made backpacks from Victorinox and other companies in the 39 - 50 dollar range. Sometimes they put their 60 dollar laptop backpacks on sale for 19 bucks at off-season times (although right now, everything is hiked up for back to school shopping).
I am a bag junky, so one doesn't do -- I sort of switch out what I need when. My everyday messenger bag is a Timbuk2 Commute -- love it, super safe, water resistent, and loads of pockets -- but they are not cheap. You can get used ones on eBay for much less at times, but they retain their value even used since there is basically nothing that goes wrong with them.
I also have an Oakley laptop backpack that I bought at an Oakley outlet store for 30 dollars -- its awesome, and is big enough to put my dSLR in as well...
Another option is to get a sleeve (amazon has tons of them in the 19 dollar range) and any messenger or backpack you like. In many instances, this is the best option for a lot of people, since an older plain old bag will not scream "Steal Me!" like some of the fancy new computer bags. Just google "Macbook sleeve" and lots will pop up. I prefer Be-EZ sleeves because of their really strong support and safety factor, and the fact that they don't smell like neoprene/burning tires.
One warning to those who do international travel, go through train stations, or smaller european airports....nothing screams "Laptop Aboard" more than any bag that has an iPod slot and headphones hanging out of them; or a fancy laptop backpack with any brandname on it that is known to make only laptop bags (i.e. brenthaven, MacCase, etc). Keep your eyes on those 100% of the time.