Yu Darvish was worth about 25 WAR for the first 9 seasons in the U.S. coming over at age 25. One could easily see a contract in the $10-$12 million per WAR range now, and 7-9 years from now $14-$15 million. $300 million is not really that outlandish even if you assume he has a TJ surgery and several mediocre seasons somewhere in 9 years.
It may seem expensive, but it is really not as there are not a great deal of pitchers who can average almost 3 WAR over a decade.