There are several elegant modern prefab green homes out there now (see Living Homes, Marmol Radziner, Glidehouse) but they tend to be rather expensive. Romero's LV Series homes are panelized kits that are among the most attractive prefabs I've seen and by far the least expensive (assuming you get a good deal on the assembly or do some of it yourself). A 1400-square-foot 2-bedroom home sells for $45,000 and
is shipped on one flatbed truck. Has Habitat for Humanity seen these?
I'm writing this from my crowded home office lusting after the LVM studio cube that would look great in my back yard above the high water mark for less than $24,000. Some assembly required.