Basically, teams paid approximately $8 million per win through free agency this season. Therefore, Porcello's 5.2 WAR is worth $41.8 million to the team. Porcello would not make that much as a free agent for 2 reasons.
One, free agents get their worth in a contract through the length of the contract, not through $/year. Your really good free agents, like Price, sacrifice what they are actually worth per year to get the 7+ year contracts. Price may be worth $35 million this year, but he likely will be worth far less than that by year 7. If he were willing to sign a one year deal, he might have received a $50 mil/one year contract.
Two, while Porcello was worth $41 million this year, there is no guarantee that he will continue to be worth that much next year or any year after that. Regression due to aging is factored into free agent contracts, which sort of goes along with point one.