Here's a great looking little tree house plan that was offered up in a 1963 issue of "Woman's Day" magazine. Given a little tweaking, and enlarging (larger dimensional lumber for the deck, a taller/wider A-frame, and so forth) this could make for a very nice, affordable, vacation, or weekend, retreat. As long as you had two trees to connect half of this cabin's supporting members, the land below wouldn't so much matter, so chances are you could buy some less prime land (CHEAPER, in other words) and easily make this work.
I'm off to find this back issue on ebay, as I'm really curious to see the plan set...although one could easily hash it out just by looking at this photo, or come up with a reasonable facsimile.
-Derek "Deek" Diedricksen