If you're a budget-conscious shopper, your best bet is this Cabas Backpack. It's a wonderful item, while not very costly. Another strong contender for your wardrobe is this NO KA 'OI Backpack. While the price of this piece is competitive, it will definitely attract a lot of attention. You might also want to consider this Calvin Klein Jeans Rucksack. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
If you're on a hunt for a luxury item, then what you're looking for is this Maison Margiela Backpack. It's more expensive but worth your money. While you're at it, also have a look at this Elisabeth Weinstock Rucksack. Yes, this item is expensive, but it does not mean it's not worth it. And one more item you might be interested in is this Chloé Backpack. 'Cause you deserve nothing but all the finer things!