Roofs are a sandwich of fiber panels over styrene insulation; effective & cheap. Rooms are larger than appearing from
outside and include 2 bedrooms, a fair size bath. The kitchen, not visible, includes a stove. Floor area is 60 m2/ 646 sf.
I  was a very festive event, attracting thousand of onlookers, Mexico City movie stars and every politician worth half a vote,
including the helicoptering governor, the president´s wife, and several critics.

Christianity Today - A review of Habitat for Humanity - How to Build Homes Without Putting Up Walls
Habitat for Humanity - The US home page
Hábitat para la Humanidad México - the mexican affiliate web site
selecting the future habitants.
Here´s a look at the other side of the building spectrum:

This is a sample of a local bottom line development. Asking
$280,000 pesos, 2 bedroom, 1 bath, tile floors, a little bigger
than Habitat, and standing almost empty for more than a year.
I´ve seen similar casas in Veracruz city for less than 180,000
pesos. That´s probably why the ones in San Andres are
unoccupied.

The habitat homes were "sold" at 600 pesos per month for 8
years. No interest.  Roughly 60,000 pesos.

Still, more than 2/3 of tuxtlas employees earn less than 800
pesos week, which makes functional home ownership a
dream for most.

Habitat is an evangelical christian ministry with an egalitarian mission - decent housing for everyone,
regardless of creed, race or location.

This Tuxtlas event drew at least 1500 volunteers, most provided and imported by sponsoring
corporations, but also a smittering of altruistic local helpers, especially albaniles (masons) , who all
had to learn a new trick or two.

Local albañiles had to learn a new trick. On site manufacturing of cored adobe block. Considerably more
insulative and functional than the solid concrete blocks in common use in most of Mexico. Habitat brought in
the block presses and turned out blocks faster than tortillas. The holes are for passing electrical and
plumbing circuits and some rebar reinforcement where needed. The homes, except for foundations, use no
formed concrete.
Habitat for Humanity - 60 homes in 5 days
Ranchoapan, San Andrés Tuxtla, Veracruz June 5 -10, 2005