If you're a frugal shopper, your best bet is this Even&Odd Backpack. It's a wonderful piece, while not very pricey. Another strong contender for your wardrobe is this Michael Kors Backpack. While the price of this piece is rather reasonable, it will certainly attract a lot of attention. You may also want to look at this Fiorelli Backpack. This item is not expensive either, but it will make a difference in your wardrobe.
If you're after a bargain, look no further than this Marsèll Backpack. This item doesn't cost the earth, but definitely deserves a place in your wardrobe. It's also worth checking out this Will Leather Goods Rucksack. This piece is extremely popular and, just as importantly, not very expensive. Alternatively, opting for this Frye Backpack is a nice choice as well!
If you want to splurge, then what you're looking for is this The Row Backpack. It's more expensive but worth your money.