The price to a buyer will probably be in the $100/sq-ft to $1,000/sq-ft range. At the upper end of this range a 1,000 sq-ft space would cost $1 mil.
This article suggests that the cost/sq-ft would be around $337.
In a normal single family home the cost of building the house and the cost of the land are roughly the same. If we can get a single family seastead where the cost of the house is about the same as on land and the cost of the platform is roughly equal to this, then we can be about the same as a regular house. On land a house will have property tax, and the seastead will have extra maintenance costs, if these are roughly equal then the total cost of ownership can be comparable.
Ideally someone would be able to sell a regular home and then use the money to buy a single family seastead. Depending on the actual cost of a seastead some fraction of the market would actually be able to do this. If we can get the price down to where even the top 1% have houses that cost more than seasteads we should do well.