He's the best pitcher in baseball. He's won the ERA title three times already. He's about to win the Cy Young for the second time. He CAN strikeout many batters, get a lot of groundball outs, get a lot of fly outs when he needs it but he's NOT worth $300 million in my book. He barely plays in 20% of his team's games. Which goes back to, because he's a pitcher, he's NOT going to have as much of an impact as a position player. The first 5 years of the contract will still look pretty good since he's going to enter his prime years and even then, it's NOT even a guarantee that he'll keep up his pitching dominance. A lot of guys pitch well before they get their big money but after they get this contract, they struggle. (Ex: Barry Zito, Justin Verlander, Mike Hampton, etc).
He's worth about $160-$170 million over 8 years but $300 million is just too much for a pitcher. No one is worth that much money because of all the things that CAN happen that could make a big contract look bad.