If you take Lester at his word, he is willing to take a discount. But what is a discount? A $16-$18M deal would be well, well below the market for a quality innings eating starter. I think the 25M AAV might be on the high side, but not by much. Actually, for that matter - I am pretty sure the price is not what is being haggled over. I suspect the two sides are in the same AAV ballpark. We know what the comps are, and we know that the Red Sox play in the deep end of the revenue and payroll pools, and we know the fans and media would (totally correctly) pillory them for pinching pennies. The question is years, both the number and the guarantee. I would not want to go 5 years on any pitcher. But if you have the resources the Sox have, and if you want to win an auction - it helps to assess the worst case result and see if you can live with it within the big picture. For me, Lester's worst case in the next 5 years basically ends up being 2014 Jake Peavy, perhaps a bit less. Is that player worth $24 million in the 2019 salary universe? Probably not - but it would not actually be that much of an overpay if Lester maintains his flawless mechanics and strong track record of durability. If the Red Sox could get 1000 innings of (on average) 2013 Regular Season Jon Lester production over 5 years, 24M AAV would actually be quite reasonable.
I think if there is a real sticking point, it is years - either years the Red Sox don't want to give, or years the Red Sox want to put options on. For me, if I were Cherington, something resembling the Ortiz extension in terms (but longer) would make sense. 4 years, 5th year a vesting option, 6th year a pure team option.