I recently finished builing a tree-house for my two sons. I had always thought that having a tree-house would be "cool", and as our house has a huge liquid amber tree growing right outside our front porch I couldn't resist letting my "inner tradesman" run amok. (Actually its TOO close to the house - one day I'll have to get it removed before it damages the house).
Luckily my eldest son was keen on the idea, having enjoyed reading the "magic treehouse" series of books (written by Mary Pope Osborne) in pre-school. The materials cost around $500 altogether, mostly spent on the wood decking and handrails, plus some screws, decking nails and poly rope for the ladder. Luckily I already had some suitable wood lying around to use for the main beams and floor joists, and had all the required tools already.
Construction took longer than expected (doesn't it always) - around 5 days all told, spread over several weekends and a couple of days off work (the company I work for allows you to take up to half of your "sick leave" entitlement as "personal days"). I went for a simple "open" architecture - just a floor and safety rails (with fly-screen glued and nailed to the rails for added safety). As my two boys are aged 6 and 0, they'll be able to enjoy playing in it with their friends for the next 15 years, so the cost works out at only 32c per week per boy ;)
When the boys are older I'll be able to increase the "fear factor" by adding a "flying fox" (zip-line) which is available as a kit for around US$80 [link]. Hopefully the tree doesn't do any more damage to our house in the meantime - it had already put a small crack in the brickwork at the front of the house before we bought the place 3 years ago, and we've had a couple of visits from the plumber to clear tree roots out of the sewer pipes. Hmmm - perhaps the total "cost of ownership" will average out at a bit more than 64c a week.