PunditFact ruled that claim "Pants on Fire," finding only six countries with a GDP lower than that predicted for the fight: $400 million.
On the other hand, PunditFact gave Cokie Roberts a "Half True" rating for a claim she exaggerated by over 9,000 percent. PunditFact gave Roberts credit for her underlying point, that the risk of getting murdered in Honduras is greater than for New York City.
Apparently Lampley has no valid underlying point that the Mayweather-Pacquiao fight would generate a great deal of revenue.
You be the judge.
Update May 3, 2015
While researching and wondering how Lampley ended up with 29 countries producing a GDP under $400 million, we noticed a perhaps-coincidental statistic: The World Bank's 2013 GDP rankings have 29 countries with a GDP above $400 billion.
Lampley's claim may have started with this statistic. After mixing up millions with billions and mistaking the top of the list for the bottom, Lampley's claim makes perfect sense, in a way.
Correction May 4, 2015: Fixed spelling of "Pacquiao."