If you happen to live near a wood, it is totally free.
It is not as hard as it seems if you know a little about wood.  And it depends what bow you intend to make that affects the level of difficulty.  There are some great lists of good bow woods out there, but the best in Europe are Elm and Yew, in Asia, mulberry, in America, Hickory, Osage and a couple of others.  Birch and ash are also quite good bow woods.  One cardinal rule, the only one I can think of right now, is not to cut into the sapwood on the back of the bow (unless you are making a backed composite.)  The reason is that cutting down to the grain means that splinters will rise up in the wood, ie, the bow will develop a fret, crack, whatever you want to call it, and break, if not immediately, then pretty quickly.  If there is anything you don't understand here, then look it up somewhere, there are lots of resources out there.  Some good books are Longbow by Robert Hardy and the Encyclopedia of Native American Bows, Arrows and Quivers by Jim Hamm and Steve Allely, I think.  Ask if you need and more pointers, hopefully someone here can help you out.

go here, they have all the supplies, books, videos, kits, and just about anything else you'll need to make your own archery equiptment:

http://www.threeriversarchery.com

It can be as cheap and has expensive as you want to make it. My first bow was cut for free and i did all the work with a scrhade sharp finger knife. So i only had at the most $20 dollars in the knife. Take my advice, a good bow can be made with just a knife.