You are doing the right thing...
Five years from now you won't give the money a second thought. What you will always remember is helping a good kid do something productive. The memories will be worth 200 times what the wood costs...and heck in five years the price of wood will have doubled and you will be laughing at how you thought the wood was "expensive"!!
By the way I used to do a lot of training in the arena of safety for railroad workers. Teaching is a misnomer...teaching is learning. To teach you must concentrate on each task separately and think of a way to intelligently present it to someone else without confusion. To do that takes a lot of thought, patience and understanding just what has to be accomplished, and some of the hazards. As I said, its learning in disguise.
Still don't forget to have fun...with the project and your neighbor.
I have no intention of traveling from birth to the grave in a manicured and well preserved body; but rather I will skid in sideways, totally beat up, completely worn out, utterly exhausted and jump off my tractor and loudly yell, "Wow, this is what it took to feed a nation!"