My friend and I built 2 so far.No cost involved yet except for some fasteners.
The 4' by 4' frames were 2'' by 3" angle iron we found. I welded them together and bolted scrapped 2x6 treated deck boards to them.We fastened them into the crotch of a couple of oak trees about 15' up.
The ladders I built out of scrapped garage door channels.You know, the ones the rollers ride in and they have the 90 degree curve at the top.The rungs were cut out of more channels and welded to the upright channels.
We haven't built sides or roofs on them yet.We'll just hang some camo burlap on the sides until then.