Monaco has a tiny territory and is bursting. To expand, it seems to have nowhere to go but the sea, à la the Netherlands. And this seems exactly to be the plan: filling up parts of the Mediterranean sea on the shores of Monaco at a cost of €5 billion, to deliver 275,000 square meters of land. This is about US$ 3,000 a square foot. Monaco can do better than that.
The area around Monaco is quite hilly, so I suspect the water is not shallow. This makes it particularly difficult to fill. Also, there may be environmental issues with marine life. I think it would be much simpler to simply expand into existing land, i. e., buy it from neighboring France. And France should be happy to sell.
France should be able to get a good price for it. And it is not losing much. Monaco is a tax tax haven, but not for French nationals. Indeed, after France embargoed Monaco in 1963 because of tax cheats, Monaco had to give in and let France tax its citizens living in Monaco. So no tax revenue loss for France, a apart from the non-French residents that would fall out of its jurisdiction.