Black Canvas Backpacks from Zappos
If you're a frugal shopper, this JanSport Backpack will fit the bill. It's a popular choice, while not very pricey. In this price range, you can also consider this Kipling Backpack. It's also a popular item and won't break the bank. Or you can choose this Timbuk2 Rucksack - be sure you'll be making a stylish addition to your wardrobe.
For something in the middle of the price range, look no further than this Tumi Rucksack. This item doesn't cost too much, but absolutely deserves a spot in your wardrobe.