By that math, had he stayed in Japan, his opportunity cost would be about $18,000,000 per year. While Opportunity cost can be a dollar figure, it can be applied to any decision be it monetary, time, materials, or otherwise.
Comparing the two leagues, the next problem that arises is the marketability of the teams. In the NPB, the teams are often owned by local businesses and multiple teams may play in the same city (Baseball in Japan: The Old Ballgame). This severely hampers the earning power of the franchises and the players alike, because of market saturation. The saturation comes from the high number of teams in a particular city outweighing the demand of that city's baseball enthusiasts. The principle at work here, Supply (the number of teams in cities) and Demand (the number of fans willing to watch) plays a large role in the discrepancy of salaries between the two leagues. (Baseball in Japan: The Old Ballgame) Were these teams to spread to other cities to expand their markets like MLB has, it could significantly raise the profits and in turn the salaries as well by expanding the base of people who could see games in person. Those expanded fan bases lead to more ticket, merchandise, and concession sales, driving up revenue. That increased revenue leads to more money trickling through to player salaries. This is a perfect example of how a money flows through an economy. The more fans, or households, teams or firms reach, the more money comes into the team. That in turn leads to more money being able to flow from the teams back to the players, who fall in the household's category.
Facing the problem of lower earnings potential, in the late 1990's players in Japan began to look for way to take their talent to America.
One loophole found early on to circumvent the Japanese ten year contracts, was to announce their retirement from the NPB, and market yourself to MLB in the United States.
