According to wikipedia Guam has a GDP per capita of $30,500 and American Samoa has an even lower GDP per capita of $13,000 so 5:1 is closer to reality for the US.

That said. The US is not the only country to oficially use the US dollar. Among others, Ecuador has a GDP per capita of just under $12,000 and uses the US dollar, Palau $16,000, East Timor $5,500, and lowest are Marshall Islands and Micronesia both with GDP per capita under $4,000 bringing the ratio closer to 1:18.

Also there are plenty of counties with even lower GDP that uses the dollar unofficially, so I don’t see a problem with a high GDP per capita ratio.

Edit: Equatorial Guinea is also an outlier with high GDP and low population. We could arbitrarily decide to include San Francisco in the US example with a GDP per person of over $100,000 bringing the US ratio up to 1:7 and the US dollar ratio to 1:25

It's not as if most people are rich in Equatorial Guinea. It's an appalling outlier in terms of inequality, even on such an unequal continent as Africa.

"Only about half of the country’s population has access to clean drinking water, overcrowded living condition is—surprisingly given the country’s low population density—rampant and very few children enjoy the advantages of urban life such as education, medical services and recreational facilities." https://borgenproject.org/equatorial-guinea-poverty/

I tried to find data about GINI for the country but the World Bank and the UN don't have anything. http://hdr.undp.org/en/content/income-gini-coefficient

Is state-level GDP per capita the correct model to use? Why not compare, for example, city-level GDP per capita? After all capital and workforce are free to move between cities not just countries.

(Plus the whole fun where American member states are not entirely sovereign, while EU member states are countries in their own right, and in the case of e.g. Germany are themselves federal nations with multiple constituent states)

Well yes. And I don't really understand why a common currency between rich and poor countries is a problem. But people are saying that it won't work. I suppose it won't work if you depend on the government to be able to control the money supply to fool people into thinking they have more or less money than they really do. But is that really necessary?

But Equatorial Guinea is an outlier.