Cost of making up a rumor : 2c
Cost of proving a rumor is false : $$$
Cost of disseminating that proof to say 90% of people fooled: $$
Cost of persuading those 90% that your proof is more reliable than the rumor: $$$$$
Cost of proving a rumor is false : $$$
Cost of disseminating that proof to say 90% of people fooled: $$
Cost of persuading those 90% that your proof is more reliable than the rumor: $$$$$
Cost of updating the rumor just enough so the proof it was false no longer quite applies: 1c
Also, in addition to dollar costs there are time costs which similarly favor the rumor maker over the rumor debunker.