If you're looking for an item in the middle of the price range, look at this Marsèll Backpack. While the price is very reasonable, this item is an easy wardrobe update. You can also consider this Will Leather Goods Rucksack. It's an attractive option both price and quality-wise. And do check out this Frye Rucksack! It's a very popular item and good value for money.
If you want to splurge, then what you're looking for is this The Row Rucksack. It's more expensive but worth your money.