If you can’t spend much, why not look at this Aldo Rucksack. It's a great piece for a reasonable price. Another strong contender for your wardrobe is this Rains Backpack. While the price of this product is not too high, it will certainly attract a lot of attention. You might also want to consider this Herschel Backpack. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
If you want to splurge, then what you're looking for is this Chloé Rucksack. It's more expensive but worth your money. Why not take a look at this Fendi Backpack too? Expensive, but worth every penny. And how about the luxurious MCM Backpack? Yolo, right?