That's a good question. It has to do with what's the definition of a Seastead.
Let's say we have 50 people living on a group of floating houseboats, linked together by a $150k dock. Does this constitute a seastead?
How is it any different from a land based set of houseboats. Here in Portland, Oregon, there are something like 3000 people who live afloat in floating communities. Are they seasteaders?
In my view, the key difference in a seastead is not to go "stealth", but to create an autonomous city-state. To do that means to license and obtain some official recognition. The prerequisite for this is to be very safety conscious. We can't be licensed as a passenger carrying "ship" without conforming to some very rigorous standards that there be no loss of life. Even after floating, any accidents would be the end of the project. For that reason, I think the engineering and planning needs are high. But that's a different scope project than you are suggesting.
In my mind, $200 million or so is about minimum for getting an autonomous community afloat, and a good quarter of that is safety and engineering.