Oh it was not stupid for Greinke (he's a really good pitcher). It was just the market he entered into. Lots of money chasing relatively few players. The draft picks are a far bigger deterrent than the salary.
Here is the thing - Lester might want to trade some salary because he is in a place where his family is happy. He owes the Red Sox nothing - it's business. But that has to come with some additional security - it's the tradeoff people make, perfect sense. Now Lester is a $25 million a year pitcher in the open market - quality #2, fringy #1, no injury history and a sure bet to spin 200 innings at a pretty good to really good level, and most of the teams are swimming in cash like Scrooge McDuck. He may not get it from all 30 teams, but he will fit many teams' budgets at that price.
He can offer a discount - but it has to be a discount from a fair open market price for him, not some sort of number dropped from the sky which seems emotionally satisfying but will not result in anything but some extra money to spend on Liverpool FC. $20M is a completely worthy number. Could he be had for less? If he feels good enough about his life, sure. But he certainly does not owe the Red Sox that, and asking for $20M is a considerable discount already.