If you're looking for an affordable item, your best bet is this Gap Trench Coat. It's a wonderful piece, while not very pricey. Another strong contender for your wardrobe is this Tommy Hilfiger Trench. While the price of this piece is rather reasonable, it will surely attract a lot of attention. You might also want to consider this River Island Trenchcoat. The price is very reasonable, but the effect on your wardrobe will be pretty fantastic.
For something in the middle of the price range, look no further than this Trench. This item doesn't cost too much, but absolutely essential in your wardrobe. Another nice option is this Diane von Furstenberg Trenchcoat. It's an attractive option both price and quality-wise. And do check out this Herno Trench! It's a very popular item and good value for money.