Thank you for your interest in Photon Flight and/or Turtle Airships.
Turtle Airships has suggested fielding several differing sizes and capabilities of airship types. Obviously, the costs for doing so will vary widely.
RE: an "absolute minimum size";
It is certainly possible to build a small model of a Turtle Airship using very simple materials such as balsa, ripstop nylon, etc., along with simple solar cells, electric motors and propellers Such a model could be as small as 6-10' in length. This could be constructed for a few hundred dollars
However, the materials that are used in the Turtles' hull are heavier than expected for a lighter-than-air craft, and so any airship that is more representative of the Turtle airship has to be larger in order to compensate for this.
The next step in size would use materials such as rigid plastics for the hull panels instead of the true aluminum/carbon fiber honeycomb of much larger, true prototypes, and carbon fiber for the framing of the craft. It would also be desired to increase the amount of, or the type of, the solar cells used so that they more accurately reflect those to be used on large airships. A "minimum" size would be close to 25' in length, and cost as little as $2,000. This would still qualify as a "model" rather than a "prototype". This size craft is very close to our UAV craft planned.
A craft that is twice the size of the above (45' long) could carry a pilot.
We intend to fly a demonstration airship around the world, carrying 3-5 persons. This craft will take a few hundred thousand dollars to build, because of it's size and because it will use the real hull materials, thin film photovoltaics, jet engines, and other features of the design.