I get your point and don't necessarily disagree.
The only catch here, and it can't really be factored in by any mathematical method, is that Betts signed an extension with the Dodgers. He did not get to free agency. That's the catch.
And it was a relatively team-friendly deal, when you factor in the deferrals.
So this was a big extra benefit to the Dodgers. And if the Red Sox had any aspirations of bidding on Mookie in free agency, which no one really knows about, such aspirations were obviously snuffed out by this.