If you are on a tight budget, your best bet is this Even&Odd Backpack. It's a great product, while not very costly. Another strong contender for your wardrobe is this Herschel Backpack. While the price of this item is not too high, it will certainly attract a lot of attention. You might also want to consider this Fjäll Räven Rucksack. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
If you're looking for a happy medium, we've got your back with this Moschino Backpack. This item is bound to become a total wardrobe staple. You might also want to look at this Orciani Rucksack. It's an attractive option both price and quality-wise.