According to The Athletic's Rustin Dodd, Jay will earn a $3 million base salary with another $1.5 million available in performance bonuses.
Jay enjoyed a productive season with the Chicago Cubs after signing a one-year, $8 million deal prior to the start of 2017. His .749 OPS was his highest since 2014 and, according to FanGraphs, Jay's 1.6 WAR was sixth-best among Cubs position players.
In June, Cubs manager Joe Maddon compared Jay to a valuable bench player in basketball.
"It's kinda like John Havlicek—the sixth man in basketball becomes famous," Maddon said, per CSN Chicago's Tony Andracki. "He's the sixth man here. You can pop him in there and it's like instant offense. You know something good possibly can happen. You know he's ready."
As Maddon attested to, Jay was very good when coming off the bench for the Cubs in 2017. He batted .358 and had a .435 on-base percentage in 62 plate appearances as a substitute, according to Baseball Reference. He also hit .325 exclusively as a pinch hitter.
While the Royals likely envision Jay as an everyday outfielder, the fact he can be effective off the bench during games adds to his value.
Jay won't add much power to Kansas City's lineup. He has 33 career home runs since 2010 and his career slugging percentage is .383, which is tied for 339th among qualified hitters during that span, per FanGraphs.
Jay is able to consistently get on base, with his .374 on-base percentage in 2017 a personal best. He's also a solid baserunner, with his 51 career stolen bases not an accurate reflection of his contributions on the basepaths.
His defense may be a different question. According to FanGraphs, Jay had an 11.5 ultimate zone rating per 150 innings when deployed in left field. That number dropped to minus-6.1 and minus-4.9 when playing center and right fields, respectively.
Jay wasn't one of the premier outfield free agents, and adding him to the lineup won't drastically improve the Royals' offense. But he's a solid value-for-money signing.