First, all houses are boxes. If it is stick built or modular it is a box.
As to floor plans being limited, I designed my own. he only limitation was placement of the basement steps due to a 9'3" basement wall (headroom) and the fact that SWWBO shot down my first 20 floorplans. /forums/images/graemlins/shocked.gif
http://www.smihomes.com/ourbenefits.htm
I saved by doing all my own plumbing, electric, heating and finishing, basicly they set 2 real big boxes and drove away. All of the plumbing was stubbed down, upstairs elec was dropped down, registers and elbows were in place, all of the trim was included.
I am thinking for the house itself without basement it was 40-45 bucks/sq foot off the top of my head.
There are HUGE differences in modulars as there are in contractors. Mine is a middle of the road fancy wise. It is overbuilt but goodies were optional. I have a friend who has had 2 built. One colonial (Ritzcraft homes) higher end, and a chalet type, dont know the manufacturer.
The only way you can tell that they are not stick built is that the marriage wall is 6 inches thick instead of being a 2x4 studded interior wall.
Financing was a pain because some banks are still modular=trailer thinking. Not all and it is changing. However the real problem financing is in having more than 7ac. Banks hate that.