Huh? I didn't say Johan Santana. Santana never hit free agency, the Mets traded for him and then extended him.
The examples I gave were Cliff Lee, CC Sabathia, and Mark Teixeira.
My logic for signing guys like that is simple. If an elite player regresses they are generally still above average and worthy of a spot in the line-up. If a mediocre player regresses they end up borderline starters, and sometimes guys who belong on the bench.
Mark Teixeira is a perfect example. He hasn't performed as well as the Yankees would have hoped, or what they're paying him for, but he has still been acceptable, and worthy of a 3-5 spot in the line-up.