In this case, each person paid for the entertainment of watching their friends eat shit.

Not to overanalyze the joke even more, but:

Eating feces is not $100 worth of entertainment by any rational standard.

No rational person would spend $100 to watch his friend eat feces.

No rational person would accept $100 to eat feces.

No rational society allows someone to either eat feces or pay others to do so, for public health reasons if nothing else.

I mean, if you went to an unhoused person and offered him $100 to eat feces, you'd get arrested. And you'd deserve it. Because even the United States isn't quite that bad yet.

(And this is not a hypothetical. People do those kinds of things. There are unhoused people I know from Food Not Bombs who refuse food from strangers because too many of them have gotten adulterated food. And most of them have stories about people offering them money to do degrading things.)

So this $200 in GDP represents an activity that's injurious to public health, morally bankrupt, and leaves everyone participating in it worse off.

But from the Economics 101 worldview, the economists created $200 worth of entertainment, because both of them were willing to pay $100 to see each other eat shit and that means, by definition, eating shit was worth $100 in entertainment.

Which makes the punchline an even more vicious satire of capitalism and its bullshit metrics than it originally appeared.