[...]of course, California is rich because of its membership in the U.S. of A., but that's another thing, eh?
I expect this is true for much of the country, that our wealth is in part a product of the fact that we
aren't a collection of 50 countries.
There was an article in the paper yesterday about how some US states are starting to get concerned because their neighboring states have varying levels of control on the coronavirus. Whether it's different requirements about wearing masks and 'social distancing', or just cultural differences, people in one state may be more laissez faire about it, but they're still free to travel as they please. It's more complicated than that, though. Because our state borders are wide-open, people along every state border in the Union travel across them on a daily basis, living in one state and working in the other, or their families spread out. States have varying sales taxes, so people living at the border of a state with higher taxes on retail purchases routinely go next door to shop, probably without even thinking about it. In some places, you might not even notice you've crossed into the next state.
So in an economic sense, the US states aren't just
next to one another, their economies overlap willy-nilly. If you tried to make an "economic map" of the US, you probably wouldn't even be able to pick out all of the states' political borders. And that's without taking into consideration the distribution of Federal funds for things like our interstate highways, which are literally invaluable to commerce. Imagine if Wyoming had to pay for all of its own highways. What would that do to the economy, not just of Wyoming, but neighboring states? How much of the commerce to and from Nebraska and Idaho uses Wyoming's interstates?
Also, our international trade comes through a handful of big ports. iirc, Houston, New Jersey/New York and Los Angeles/Long Beach are the super-duper ones, but we have some others. I don't know how much of the value of that international trade ends up in the GDP figures of those states. I don't know how the EU handles it, vis a vis the ports of Rotterdam, Antwerp and Hamburg, which are their super-ports (in terms of total traffic, Rotterdam and Antwerp are actually bigger than any of our individual ports - and if you're wondering, yes, the East Asian ports blow everybody out of the water). Unless the Dutch, Germans and Belgians are actually consuming all of those goods themselves, there must be some rules about moving freight across European national borders by truck and train without the goods being so expensive at their destination that almost nobody will buy them.
I suppose the relationship between those 50 hypothetical independent nations could be whatever they wanted, but I suspect that the collective, overall wealth of the 50 states would drop. For comparison's sake, the GDP of the EU is close enough to ours to call it close enough, 18 trn to 20 trn, but they have more people.