Unless you are doing stuff off the books, we know exactly how much money is spent on a yearly basis, it is why every entity: person or business, has to file taxes each year. The amount the government spends(local, state, federal) is also well documented. The total amount spent is the worth of the goods and services being produced. While it is after all an estimate, it is pretty damn close to the actual figure, the $20 you bet your buddy on a game still ends up being spent, and Walmart still files their taxes.
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