I paid $50 for my Targus. It has everything I could possibly need. I have my camera and charger, food, water, pop, utensils, power cord, mouse, DVDs, headphones, room for little doodads and of course my lappy in it. And I still have room for speakers when I find some I like. It already weighs a ton. I can't imagine getting anything bigger. Besides, as far as I can tell, you're just paying for bells and whistles like more robust handles and rubber feet, fancier colors and the like when you buy one of the more expensive models. I don't see the point. Paying $50 was painful enough. I can't see paying 70 or 80 bucks for a backpack.
For those looking for a backpack, I'd suggest getting the least you can get by with and spend the extra money on your lappy. You'll probably get tired of lugging it around anyway and start leaving stuff at home that you think you don't absolutely need.