The trench coat is a timeless design for fall and spring that also does an excellent job of protecting you from the inclement weather. Wear your trench over everything from casual outfits to more sophisticated pieces.
If you're a frugal shopper, your best bet is this Topshop Trenchcoat. It's a popular item, while not very pricey. Another strong contender for your wardrobe is this Topshop Trench. While the price of this product is competitive, it will definitely attract a lot of attention.