If you want to know Kimbrel current value, you need to start with the package the Sox gave for him two years ago: Manuel Margot, Javier Guerra, Carlos Asuaje and Logan Allen. That was the value of Kimbrel with three years of team control, his value with only one year should be far less than that. But he's one of the top three RP in the game, you might get something better if he's traded during the season, by the way, I don't think the Red Sox trade Kimbrel, I bet they will try to extend him.
Regarding the Chapman, the Yankees basically got Gleyber Torres, because Adam Warren was just a throw-in (he was terrible the first half of the year), Billy McKinney prospect value was very low and Rashad Crawford, which value was never high. I know this deal was a huge win by Cashman and the Yankees scouting team, but it was a win-win at the end, the cubs got their first WS in a century and the Yankees got the prospects they wanted. At the end of