This is a 8' hexagon house. The foundation is 12 4" round posts. The posts can be harvested locally from small trees, bamboo, steel posts or cheap alternative. Posts will be wrapped in multiple layers of plastic bags before being planted in the hole. The top of the post is girded with 1x6 rough sawn or equivalent which will also make up the rafters. Six 4x8 sheets of the cheapest local material to deck the roof. Rolled roofing, plastic, etc. to make watertight. There are 2 options for walls.
1. Infill: the walls are in filled with whatever is locally available and cheap .could be sandbags, rammed earth, adobe, rubble, cordwood, etc... whatever is easy and cheap (free).
2. Sheeting: 12 4x8 sheets or equivalent are what it takes to close in. 7/16 osb, Rough sawn, R-board or any inexpensive alternative. Hard pack dirt floor with the ability to add in a raised floor later. The raised floor could be made for 5 sheets and 1x6 floor joists leaving the center open for rocket stove. The door is framed with lumber then skinned in wood, metal, etc�
As you can see this design is not about what is the best material, but how the best material can be used in the design. The hexagons 166 sq. ft. footprint gives the most space for the least material. Post frame takes care of foundation issues. The design could be enlarged to 309 s.f by making an octagon, this would only increase material 20%.