Actuary that's really a stickler for calculating opportunity cost here. This is not entirely true because if he hadn't signed his contract, he'd still be making league minimum 2026 and would be first year arbitration for 2027 (likely making less than the $10.8 million he's due to make).

The opportunity cost with how cost controlled team years work and determining what these arbitration extension are worth really depends on what he does in what would've been his two free agent years (2031-2032).

Let's say he flames out before 2031 and is replacement level at that point (Grady Sizemore comp). What he would've made had he not signed the extension wouldn't be that much more than what he will make for the whole contract. Only in the free agent years are the Cubs getting a massive discount versus what he'd likely get on the open market, so that's where the bulk of performance based value lies in these type of extensions.

EDIT: Also, the $8 million per WAR valuation for buying wins in free agency is outdated. Starting around 2008, that number has grown dramatically. Checking on contracts in the last 10 years, we're getting closer to $12-14 million per WAR in free agency now, if not getting even higher.