I have an idea of a small experimental hexagonal platform of 166 sq.m / 1 786 sq.ft.
Preliminary calculations show that the construction of this platform costs 50,000 - 75,000 USD, including unexpected expenses.
This price seems reasonable. It is calculated by current prices of Odessa shipyard (Ukraine). I think the construction such a platform in China might cost even less.
The idea is to create a small "village" of 10-100 of such platforms.
I have also thought that if create a "mother" platform that will be a floating construction yard for such platforms to build these offshore, that could provide the experiment for unprecedented experience and the vision of future bigger concepts.
The costs might be reasonable comparing to 50 by 50 meters platforms that cost 15 mln USD (according to The Floating City Project 2014, p.25)
materials: concrete, steel, polyurethane foam or expanded polystyrene or similar