If you are on a budget, your best bet is this Herschel Rucksack. It's a terrific item, while not very pricey. Another strong contender for your wardrobe is this Fjäll Räven Rucksack. While the price of this piece is not very high, it will certainly attract a lot of attention. You may also want to look at this The North Face Rucksack. This item is not expensive either, but it will make a difference in your wardrobe.
If you're looking for a happy medium, we've got your back with this Moschino Rucksack. This product is likely to become a total wardrobe staple. Another good option is this Orciani Backpack. It's an attractive option both price and quality-wise.