I am in the process of putting a tree house up, myself, and also wanted to do it on the inexpensive. Go on craigslist. There are a LOT of people who have tree houses and children who are grown. They want you to take the tree house away, no charge. I'm putting up a tree house that is actually not going to be around a tree, but next to several. It's on six 4x6 posts sunk 5 feet in the ground, each have 4 sixty pound bags of cement on them. It will have a 10 x 14 deck, and the tree house itself is 8 feet square, and six feet tall on the sides, 8 feet in the peak. I'm adding windows to it (from the ReStore), and I had to buy some new lumber, but I'm going to be able to get it up for about 300$ total monetary cost, not including my time.
I'll post pictures sometime soon, I'm going to have it mostly done this week, I hope.
Look on craigslist, get cheap materials. I also have a friend who actually just drove around looking for people with big assed tree houses in their yards and put a letting in their mailboxes, saying something like, "I can make your tree house go away for no charge!" and he had more than a dozen people contact him, asking him to take it away. Free materials, mostly constructed already, win win!