I think he looks at the win distribution like:
2014.5: 1.5 (Lester RoS)
2015: 2.0 (Cespedes)
2016: 0 (Either will have left)
So he could value current year at $7.5m per win and next year at $5.625m per win. Which would be break even on surplus value. He assumes Cespedes is leaving and notes that he's replaceable with a Gomes/Fuld platoon, which is greater than the difference of the pitcher Lester will be replacing versus Lester himself. So based on his win valuations of this year > next and the WOWYs, he figured it was worth it.
That's just me trying to replicate the thought process, though. Idk.