Baseball reference has his WAR at 2.8, so does ESPN.
http://www.espn.com/mlb/player/stats/_/id/33039/mookie-betts
https://www.baseball-reference.com/players/b/bettsmo01.shtml
Betts will and should get $300 million. He is one of the best players in the league and is one of the rare guys I've ever seen in a Red Sox uniform that under no circumstance would I want to see in another uniform. He's special, and people are really taking for granted how good he is.