I agree on the Mets. I'll believe they let him walk on a 3 year deal when I see it.
That said, Alonso and Vlad aren't all that comparable. They both play 1B but Vlad is a 25 year old projecting for 5 WAR a season. Alonso is a 30 year old projecting for 2.5 WAR.
The higher a player's WAR, the more of a premium you can expect to pay. Getting all that WAR wrapped up in one roster spot is huge and teams pay extra for it.