If you're not planning on spending much, your best bet is this Backpack. It's a fantastic item, while not very expensive. Another savvy option is this Matt & Nat Backpack. While the price is competitive, be sure that this product will become a valuable addition to your wardrobe. You might also want to consider this Backpack. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
For something in the middle of the price range, we've got your back with this Rucksack. This item is likely to become a total wardrobe staple. It's also worth taking a look at this Cleobella Backpack. This product is extremely popular and, just as importantly, not very expensive. Alternatively, opting for this Backpack is a good choice as well!