Towards a more accurate (or rather, a somewhat less fuzzy) cost estimate:
Cost is always the ultimate decision factor. Let's try to hammer down that aspect of the floating dirt seastead.
Firstly: starting with the expected submerged volume of a first-draft shape with entirely handwaved dimensions, then calculating how many cubic meters and tonnes of water this figure represents, then calling this figure the amount of raw material required, is very inaccurate. Thus, we need a better estimation method.
- used plastic bottles can be had for as little as 10 bucks for 100 000 bottles, which translates to an approximate range of 95 to 160 tonnes of buoyancy and the same figures of cubic meters in volume. Plastic nets filled with recycled plastic bottles are a more or less proven construction method thanks to Richie Sowa and his Spiral Island.
- concrete delivered to your house by the marginal cubic yard reached $70 in Europe and the USA at the height of the 1999-2006 housing bubble, however the price for thousands of tonnes in a third-world construction site during or right after the global recession may differ greatly. At $10 a tonne that's a reduction factor of almost 3, and even lower prices should be available soon.
- infrastructure is not cheap either, but the higher surface, cargo and volume capacity of this design allows for less capital intensive solutions to be chosen. The mass and volume of this infrastructure should come in deduction to the raw amount of material required, in order to maintain the stead's ratio of buoyancy (within the established mechanical constraints).