There are two plant nurseries in a small town. They are called Tumbleweed and Native Roots. If neither advertises, Tumbleweed makes $80,000 a
month in profits and Native Roots makes $95,000. Advertising would cost each firm $20,000 a month. If only one firm advertises, that firm increases
sales by $50,000 a month whereas the nonadvertising firm loses out. If Tumbleweed doesn't advertise but Native Roots does, Tumbleweed loses
$30,000 a month. If Native Roots doesn't advertise but Tumbleweed does, it loses $35,000 a month. If both advertise, they increase revenue by
$15,000 each. Insofar as they grow their products from the ground, they don't have any increased costs when they have increased sales (that is, their
marginal cost of production is $0).