Oh they could easily overvalue him.
Looking at it neutrally, Duran has been worth 10.3 fWAR in the past 2 years, while Greene was worth 6.8 fWAR. Over the next 3 years, Duran could be worth 10-15 fWAR and might cost $30mill in salary.
Greene over the next 4 years might be worth 12-15 fWAR, assuming he maintains his 150 IP, which is not a given. For roughly the same output as Duran, Greene would be paid $40mill. I think BTV is projecting both like this, which is why they are equal in a straight up trade. If Boston adds in one ranked prospect (Garcia or Clarke), it should give the benefit to Cincinnati.
Of course if they overvalue Greene, they could easily hold out for more…