From what I can find it looks as if meteorites are sold based on weight and type. They use grams rather than pounds. So 18.4 Pounds = 8346.099608 Grams. Depending on the type of meteorite it can range anywhere from a few cents a gram to $50 a gram. So with a large chunk like you have it would definitely be something that you should take to an expert in person. Some places that you can visit to get a more concrete value would be school with geology department and even an auction house can help you with finding an expert in your area. Here are a few websites with information you can look over.