The cheapest will always be what you have! I have seen knives made from those $4 machetes you get at Harbor Freight (You can get two knives out of one easily.) I have some cheap, flat pry-bars that are earmarked for knife making.
If you are just starting out I would go with what you can find cheap that looks like it will work. After some practice you will have some knives on your belt and some experience under it
Then get some O2 tool steel or some AUS 8A.
Bladforums has some decent threads about knife making with nothing but hand tools. There are some videos on youtube showing start-to-finish with just hand tools.
Let us know what you make!