My husband and I built it together. I'd say it took about 30 hours and maybe $100. We bought all of the cheapest wood/plywood and used some stuff we had at home. It helped that it didnt' have to be perfect. Imperfect materials are ALOT cheaper.
Here are some pictures of my foster kitties on the inside. It's tall enough for me to walk into without slouching (I'm 5'5"). It's about 12 feet long, 6 feet wide and 6 feet tall.
On the floor is some carpet remnants that I got from my office building (for free). The black and red marks on the wall are my 6 year old step-son's version of "high-rises". It's a city theme......see the blue sky.
This is Buddy and Pennsylvania. Very sweet kitties. They were on their last day at a high-kill shelter.