I'm fine going into the 5/100 or 6/120 range for Lester. For a lot of reasons.
1. He's pretty much a lock to give you 30+ starts and 200+ innings every year.
2. In line with #1, he has no history of serious injury and he has very good mechanics so there isn't much to suggest that he's a major health risk going forward
3. Since he became a full time big leaguer in 2008, he's had one year where he failed to post an ERA in the 3's. And even in that off season (2012) his peripherals suggested he was the victim of some poor luck.
4. He's a lefty with a history of success at a park that isn't too friendly to lefties.
5. He's been nothing short of exceptional in postseason play.
6. 100/120 million might not seem like a discount but consider this:
Ricky Nolasco, the very definition of right handed mediocrity, just got close to 50 million dollars. How much would a guy with Lester's pedigree fetch on the open market?
Furthermore, with the way salaries are exploding in baseball, that 20 million AAV could very well look like a bargain in a few years.
Give Jonny his extension and move on with no regrets. This is a no-brainer to me.