Gross Municipal Product

So I finally get around to searching for a concept I had.

We have Gross Domestic Product.

We have Gross State Product.

But if I want to gauge how big a city is (and thus run additional calculations, like overall tax rates, etc), it would be really neat to have GROSS MUNICIPAL PRODUCT (GMP).

The concept already exists, but primarily in Japan where a Google-search turned up maybe a dozen hits of Japanese city's municipal products.

But I got nothing as splendid as a list or a database that shows say the top 50 cities' GMP in the US.

Anybody got a link or know if this information actually exists?