Alright I went a little more elaborate this time, and also made sure it was a standard 3'x3' surface.
I did not use:
1 T-Clip
1 Ramp
2 Stairs
1 Ladder
12 3" Tiles
12 3" Walls
7 3" rails
1 6" Wall
That's right, I had one clip left over. Total.

I have left spaces for three 12" x18" houses, one of which goes over the large basement in the upper left.

Courtyard with bridge to lower level, bridge removed.
The back end. You can see I made some sacrifices in stability (I would have liked to have a wall there) in order to save clips. You can also see some of the places I used T-clips instead of straight clips because it's all I had.
Conclusions: Neat stuff. You seem to need at least three boxes of clips for each two sets of cardboard. I'm not sure how useful it is for necromunda - even the houses don't seem to have removable windows, which makes targeting tough. I guess one could remove the windows by hand, but it would be long, hard, and probably cause more damage than it was worth.