If you think about it, the actual disposable wealth of a lot of small nations is probably less than what Cameron himself could marshal and spend if he wanted to. A nation in the best case needs to get the buy in of its people and administrators in order to spend a large sum of money on an expensive project. The budget for that project has to compete with all of the other expenses and ambitions that the nation has.
The difference is that for James Cameron's projects could rely on buying existing expertise from contractors. A small nation has to build that themselves in secret.
