Sometimes there are other considerations like buying or making sheaths for your knives, but most sport knife throwers do very well without these, I will not consider them further, but recommend you check out the link. From time to time they must be replaced. Sometimes they cost next to nothing.
Some people have access to lots of cardboard boxes which can be used to make indoor targets.
Sometimes even log butts, the preferred target, are available for no cost other than a little chain-saw gas. For others, access to adequate free materials is not possible, and wood must be purchased. Again, the cost will vary from place to place. Log butts are sometimes very inexpensive from saw mills, and sometimes they are hard to find. In my case I must purchase my wood.