For Clay Buchholz, I've always used the early Steamer 2017 projection of 1.7 WAR in 23 starts, which is just under the average number of starts Buchholz has made over the past three years. It's the 1.7 fWAR that was valued this year at $13.5 million, giving Buchholz no surplus value with his 2017 salary of $13.5 million.
Each team likely has its own proprietary means of projecting value, but Steamer is one widely cited and publicly available projection. Each front office could have a higher or lower projection for Buchholz.