If you are on a tight budget, your best bet is this Asos Knitted Hat. It's a popular product, while not very expensive. Another savvy option is this Reclaimed Vintage Hat. While the price is competitive, be sure that this piece will become an excellent addition to your wardrobe. You might also want to consider this Alice Hannah Knitted Hat. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
For something in the middle of the price range, check out this Asos Beanie. While the price is very reasonable, this product is an easy wardrobe update.