No we are not going for a floating project. Anyway, floating islands can become unstable if they exceed a particular size and will eventually break-up. And regarding the costs for filling 200 meters of sea, it will be surely more than $250 billion. A total of around 5 billion cubic meters of maters will be needed for a pyramidal structure. Even if it is entirely of sand, it will cost some $35 billion. But we need a lot of concrete, steel and other materials. So the cost will rise to many times that.
No they don't: Oil platforms (heck oil platform has dig anywhere from 4 to 12km deep and the head had to go straight (if the platform move it could break the whole drilling head and what's not, yet it doesn't happen why? because the platforms are stabilized and fixed in the location. And can you please provide more data about the costs you are talking about and how you are able to calculate them.
Also if you are going to build an Island on international water, no matter where you built it it will not be on shallow water for sure so I believe that this shoudn't be an argument to begin with