Despite an 80-game suspension last year, Robinson Cano has posted 20.8 fWAR, valued at $163.7 million, in the first five seasons of his 10-year, $240 contract with Seattle.
David Price, who is three years younger than Cano but with more health concerns, has posted 8.2 fWAR, valued at $65.8 million, in the first three seasons of his seven-year, $217 million contract with the Red Sox.
The Mariners needed to attach significant surplus value in Edwin Diaz to move Cano in the trade* to the Mets. The Red Sox would probably need to attach a player with even more surplus value to move Price.
* in the Cano trade the Mariners also received decent prospects in Jarred Kelenic, Justin Dunn and Gerson Bautista but took on the reputedly underwater contracts of Anthony Swarzak and Jay Bruce (Bruce has shown signs over returning to his 36-homer, 101-RBI performance of 2017)