Does anybody have an idea about how much it would cost to build an artificial island or floating island (seastead) per kilometer square or per mile square? How many millions does it take to start your own country, just for the physical infrastructure?
That depends on the design, you might design something in steel which would be expensive as well as requiring constant maintenance or you might design something that is cheaper yet will have a limited lifespan and need to be replaced after so many years.
Do you really want such a large single structure, maybe it would be better to link up 2500 rafts each 20mtr square to create your 1Km square island.
Your island could start small and grow as funds become available or demands dictate.