Comment by elgenie

One of the advantages of money is that the enjoyment is implicit in the exchange, with no explicit acknowledgment necessary.

The enjoyment derived by the watcher must exceed that of having $100 and using that money in the future, or he wouldn’t have paid out. The disgust of the eater is more than compensated by the $100, or he wouldn’t accept that payment and do the job.

Both coprophages have thus derived utility from the exchange. From an economic standpoint, eating shit is just a form of working.