If you're looking for an affordable item, your best bet is this '47 Cap. It's a fantastic item, while not very costly. Another strong contender for your wardrobe is this New Era Baseball Cap. While the price of this product is competitive, it will certainly attract a lot of attention. You may also want to look at this Marmot Cap. This item is not expensive either, but it will make a difference in your wardrobe.
If you can afford to spend a little more, look no further than this Dsquared2 Baseball Cap. This item doesn't cost the earth, but absolutely invaluable in your wardrobe.