Putting 25 million AAV into the context of past SP'er FA contracts in 2015 dollars (adjusted for 5% annual payroll inflation/mlb revenue growth), Only contracts over 100 million considered (except Lackey who is under and Tanaka who is over but no previous MLB experience)
Kevin Brown 105/7 in 1999, 33 mil AAV in 2015 dollars, age 34, previous 3 yr rWAR 23.5, 3 yr avg rWAR 7.8 2015 AAV/WAR 4.2 mil
Johann Santana 137/6 in 2008, 33 mil AAV in 2015 dollar, age 29, previous 3 yr rWAR 19.83 yr, avg rWAR 6.6, 2015 AAV/WAR 5.0 mil
CC Sabathia 161/7 in 2009, 31 million AAV in 2015 dollars. age 28, previous 3 yr rWAR 17.7, 3 yr avg rWAR 5.9, 2015 AAV/WAR 5.3
Cliff Lee 120/5 in 2012, 28 mil AAV in 2015 dollars, age 32, previous 3 yr rWAR 17.1, 3 yr avg rWAR 5.7, 2015 AAV/WAR 4.9
Zach Greinke 147/6 in 2013, 27 mil AAV in 2015 dollars age 29, previous 3 yr rWAR 8.5, 3 yr avg rWAR 2.8, 2015 AAV/WAR 9.6
Barry Zito 126/7 in 2007, 26 million AAV in 2015 dollars age 29, previous 3 yr rWAR10.5, 3 yr avg rWAR 3.5, 2015 AAV/WAR 7.4 mil
FWIW John Lackey 82.5/5 in 2010 , 21 mil AAV in 2015 age 31 previous 3 yr rWAR 8.6, 3 yr avg rWAR 2.9, 2015 AAV/WAR 7.2 mil
Jon Lester 130/6, 22 mil AAV in 2015 age 31 previous 3 yr rWAR 8.3, 3 yr avg rWAR 2.8, 2015 AAV/WAR 7.8
Basically the Red Sox are offering Lester 1 extra yr of Lackey money. Lackey of course had known elbow issues that had the Red Sox put in an injury clause while Lester has been a work horse and injury free Lester had a much better contract year but basically a similar 3 yr average rWAR
No real good comps of the other pitchers except in terms of age. Lester is 1 yr younger than Cliff Lee but did not pitch as well over the previous 3. Adjusted AAV is well below Lees (28 vs 22) but AAV/3 yr WAR is much higher. Of course, maybe the 3 yr average is unfair to Lester, as more weight should probaby be given to his last year which was a 4.6 rWAR, but thats still lower than Lee's 3 yr average.
Greinke is probably the best comp in terms of previous 3 yr performance but Greinke was 2 years younger
Red Sox have a fair offer on the table, and using Greinke as a comp 25 million AAV might not be crazy, but thats an upper limit. If he can do better elsewhere, let him. Other pitchers are available.