The ongoing gambling and gangster scandals in the world of professional sumo has drawn attention to the sport's rather shady relationship to money. Though sumo is often called Japan's "national sport" there is no actual law that says it is, and most people probably think it's the national sport because the main venue for grand sumo tournaments is the Kokugikan (National Sports Hall) in Ryogoku, Tokyo. However, the Japan Sumo Association is a non-profit organization and thus the money it makes is not taxed. In addition, rikishi (sumo wrestlers) are exempt from paying taxes on cash gifts if they receive them from individual supporters (gifts from corporate supporters are taxed). And there are special tax rules for rikishi that mean they pay slightly less in income tax than what the average person who makes the same amount of money would pay.
Still, rikishi earn less than other professional athletes on average. The highest rank of yokozuna (grand champion) receives a salary of ¥2,820,000 a month, which is a lot, but as one sports blogger put it, even a "benchwarmer" on a professional baseball team in Japan makes about ¥300 million a year. Top rikishi can earn about ¥100 million a year, what with gifts and endorsements and performance bonuses. And of course they win cash prizes if they do well in a tournament.
They can also win kenshokin, prize money offered by companies to winners of specified bouts. If you've ever been to a sumo tournament, you've seen the parade of kenshomaku (sashes) around the edge of the dohyo (ring) before select matches. The sashes carry advertisements for companies, and each one represents a prize of ¥60,000. The more topical the bout, the more banners there are and thus the higher the kenshokin that the winner takes home; that is, after the JSA takes ¥5,000 from each kenshokin for itself and the tax man takes about half of what's left. The reason you don't see these on TV, at least not clearly, is because NHK, which has the exclusive right to broadcast sumo tournaments, is a public broadcaster and thus cannot show advertising. They tend to pull back the cameras or superimpose something over the scene when the sashes are being paraded.