Anytime you can own or even look at a fossil in a museum, if the specimen comes with a bit of historic mythology or lore, that makes the discovery all the more exciting. This is one of the reasons that ammonite fossils are in great demand. They rare and beautiful and there are lots of alleged "magical" uses for them. Combined with the exotic name and background to the fossil these legends make fossils of ammonite great conversation pieces.
Even the name for ammonite fossils is rich with culture. The base of the name comes from the Egyptian god Ammon that had an image of a male human with horns like a goat on his head. As such, a certain breed of ram that dates to the time when Ammon was central to that culture seem to inherit much of the mystery and lore we often associate with such an exotic deity.
To get a good feel for what an ammonite type of fossil looks like, it is always a good start to visit a well developed museum or archeology institute where many fossils are preserved and displayed. The guides there can guide you genuine ammonite fossils for you to examine and explain to you a more about the locale where the fossil was excavated as well as the period of history that the fossil dates to and as much as can be known about the mollusk it came from.
However, it is possible to purchase genuine ammonite fossils on the market. But doing your homework about the genuine item still is important because it is easy for a forgery to hit the market for a "bargain price" and if you fall for that marketing plan, you may end up with a plaster of Paris fossil thinking you are the owner of a rare part of archeological history. So learn as much as you can both about the abundance and the origins of ammonites fossils and about how to judge them for authenticity.
Like any other rare commodity that is sold based on a limited market, the prices you pay to own an ammonite fossil will depend to a large extent on the size of supply and the demand. Obviously, fossils are not a renewable resource so the supply is capped at what has been found so far and what limited amount of ammonite fossil finds that are underway right now. So being knowledgeable about how much fossil product is on the market will help you determine price and value. Then you can hit the internet and other outlets to find just the right fossil to fit in your collection.