If you're in a thrifty mode, you'll appreciate this Asos Backpack. It's a popular choice and a great investment. Another good option is this Herschel Rucksack. While the price is not very high, be sure that this piece will become an excellent addition to your wardrobe. You might also want to consider this Pieces Rucksack. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
If you're after a bargain, consider buying this AllSaints Backpack. While the price is very reasonable, this product is an easy wardrobe update. It's also worth looking at this Whistles Backpack. This piece is extremely popular and, just as importantly, not too pricey. Look at this Backpack too! Great value for your money.