Well, how expensive your throwing knives will be depends on your style. I like the "throwing knife tripple set", it's a set of 3 throwers in one sheath, costs about a dollar a knife. These take a little practice to get used to, and teach you about ducking on rebounds, but now I can consistently sink them into the board. These knives are also safe to throw, because their edge does not curve and thus you don't cut your hand. However, if you want to move up to something bigger, you will be faced with a decision. The first option is to buy a hunting knife, they're about $5 at starknives.com in the other/economy knives/hunting knives and tools section
. Now this knive is wickedly sharp and curved, but throwable. Just leave the handle on and throw by the blade. THe problem is that if you slip you will severely cut your hand
. The other option is to buy one of the designed for throwing knives, like the GIl Hibben throwing knives.