Knives as well as swords, can be very collectible. Bowie knives still evoke the history of the early-to-mid and even late-19th century from the Alamo, Mexican American War, Gold Rush, and the Civil War, these sometimes-massive knives are much sought after and collected in todays market. Some of them are fairly crude in form and made in a naive manner, others are works of art showing off the skills of the individual craftsman who painstakingly built it. Pommels can be plain or ornately shaped in various forms, including horse heads, and some of the finest Sheffield cutlers made them with grips like fancy silverware. Blades are sometimes very plain, and sometimes engraved with slogans showing the owners patriotic fervor. Many from about 1849 have slogans related to the Gold Rush. These knives became a form of protection to the miners who were panning for gold as well their future prosperity and based on some of the extant knives, the size of the blade would be enough to scare away a possible robber.
Like many other collectibles certain makers have more value than others. English makers in Sheffield were well known for their quality. Some American makers are extremely rare and valuable today. Overall condition of the knife is a factor in its value also. Having a complete knife with a sheath can be very important to most collectors. Some of the sheaths are very ornate, boasting German silver fittings with a gilt border around the edges and foliate stampings as well as makers slogans. Some are plain with no fuss brass or iron fittings and no stampings for a functional but not fancy piece.
There are many fakes and/or decent reproductions out in the market today and it can be sometimes hard to discern the real from the not so real. Luckily, there are some good dealers and folks who have been sharing their extensive knowledge through publishing great reference books with examples for us to learn from. These authors on the subject have spent years studying the material and thankfully they are willing to share what they know. There are also many sites on the web today that you can see great original examples as well as some of the fakes and how to differentiate between the two.
Doing your homework can really help you from getting stung with something fake or something that has had modern work done to it.