The European Club Association is a group that represents European football clubs' interests and needs at an international level, trying to make sure clubs aren't left out of the decision making process of wider-ranging sporting interests like FIFA and UEFA. One thing that they've accomplished is a steadily-increasing payout to individual clubs worldwide for having their players participate in the World Cup and European Championship tournaments, something Napoli is currently profiting from.
According to a report released by the ECA today, clubs around the world have received their part of a nearly €60 million payout for their players taking part in the World Cup. Napoli received the most money of any team in Italy, getting some €930,000 for their players' success. The amounts awarded is determined by how many players each club had in the tournament, as well as how many days those players' national teams were part of the tournament.
Napoli had fifteen players at the World Cup that they held the rights to at the time. Though four have since been sold and another, Netherlands player Jonathan De Guzman, has been added since the end of the World Cup, as well as brand-new signing Ivan Strinic, those changes don't seem to have had any effect on the payout by the information available from the ECA, as the count is made as of the time of the World Cup.
Juventus made €878,000, and Inter Milan were awarded €708,000 for their players' participation in the World Cup. AC Milan, Lazio, Roma, and Parma were the only other Italian sides to make more than €250,000 out of the payout, though 25 Italian teams received payments. According to the full report, only five teams were given more money for the program than Napoli, and none of them are particularly surprising: Bayern Munich, Barcelona, Real Madrid, Chelsea, and Manchester United.
A total of 396 teams worldwide received benefits from their players' participation in the World Cup, making the ECA's efforts a rousing success, if not one of massive financial benefit to the clubs most involved. The ECA has a similar arrangement with UEFA for the European Championships, and the total payout for that event is expected to be €150 million. Hopefully Napoli will be quite well represented in that tournament as well.