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A few silly things to respond to here.
1. First, $20M for 200 IP of #3 pitcher in 2017 dollars is not at all overpaid. When you look at the comps, to say so is ridiculous.
2. 4/70, if that is the number is an absurd offer. Comparing it to you or my meager means of living means jack squat. I can make a terrific living out of $100,000 a year. But if the revenue I am producing tracks to $20M a year (for instance), my wonderful salary is still insulting and woefully inadequate compared to the production.
3. There are maybe a half a dozen franchise pitchers. If you think the option is to swap Lester with one of those (Kershaw, Darvish, King Felix, Scherzer maybe, Jose Fernandez maybe, David Price maybe, just going off the top of my head), then there is a conversation to be had. If it is swapping Lester with a James Shields - you are paying more money (because he will cost more) for less pitcher (more consistent by a little, lower ceiling by a lot). Otherwise, you are going to insource Lester's production - which is cool, but none of the guys on the farm will be able to produce the innings. To supplement this, you need either more filler (which you'd be paying either Lester prices or asking for Ryan Dempster) or more middle relief (which is generally a land of crappy pitchers by comparison).
4. You seem to have a very very cockeyed idea of the economics of the pitching market. Even if Lester is not an ace - and is more a 2/3 starter (which is a fair assertion), the extremely high probability that he can crank out 900-1000 innings over the next 5 years at more or less that production level gives him a HUGE leg up as a commodity over people with sexier ERA stats. Daisuke Matsuzaka was much less hittable than Lester in 2008, but when you cannot actually take your team into the 7th inning during most of the 30-35 times a year you are scheduled to go - who cares? You don't need a staff ace to win a title or dominate a season - the 2013 Red Sox proved that. What you do need is the ability to continually get into the late innings with your starters. The stat which correlates the best with winning big is how few number of starters a team needs to get through the marathon. Jon Lester is the sort of guy who is essential to that goal.
5. And this is not one of your points, but an earlier commenter. The Red Sox do not live in the market that NY and LA do - boo hoo. But they live in a phenomenal revenue market. Markets differ in both size and fan interest - and the Red Sox play in a city where wins bring in extra money. I hate the $/WAR calculation because that ignores the realities of markets - for Boston, that number is HIGH ... for Tampa that number is low ... for most other teams that number fluctuates. Lester's production is more valuable to John Henry's bottom line than it is to the Rays owner.