If you are on a limited budget, your best bet is this FjallRaven Backpack. It's a terrific product, while not very costly. You might also want to check out this Topshop Backpack. It's an item you'll love which also doesn't cost the earth. You may also want to look at this Herschel Backpack. This item is not expensive either, but it will make a difference in your wardrobe.
For something in the middle of the price range, we recommend checking out this Moschino Rucksack. While the price is very reasonable, this product is an easy wardrobe update. It's also worth looking at this MCM Backpack. This product is extremely popular and, just as importantly, not very expensive. And do check out this Y-3 Rucksack! It's a very popular item and good value for money.