I built a treehouse for Stevew jnr(s) this summer, same sort of process / consideration. Basically used off-cut and spare wood from skips and all over, spent very little on it. The design evolved out of looking at the materials available and incorporating them rather than designing and procuring set pieces of wood.
Try talking to a few builders, we had our attic converted a few years ago and I rang the bloke up and asked him for any spare wood they were removing as part of the process – some fantastic 2 x 4 roofing joists came from him and basically built the whole of the frame.
Pulled stuff out of skips and asked for wood on freecycle for the rest.
Had to buy some of the fixings – bolts in the main intersections between different parts of the ‘super-structure’ then good quality screws for the rest and some of the decking – missed out on a good load on freecycle, could have waited but wanted to get it finished.
Good advice about planed vs sawn wood, saves a lot of cash. I borrowed an electric plane from a mate and planed anything that needed it – took up a lot of time but saved lots of cash too.
I do not host any piccies on the tinterweb so can’t upload any – unless there are other ways.
Most of all enjoy it and remember, its for the kids, not dads – something I found hard to accept when it was finished and I had to hand it over…. very sad day.