Regarding the Betts extension. If you take his projected 2019 arb value of $18.7M and let's just say $25M for 2020 (his arb value was $10.8M for 2018 so this seems fair) and add $40M/$42M for two free agent years (used fangraphs estimation tool for a 5 WAR player) , that comes to a total of $125M for 4 years. If you apply some sort of discount rate (like 5%), that comes out to $115 PV. So perhaps a "opt-out period" of 4/120 works, (27.8, 29.2, 30.7, 32.2) which gets more "value" in his hands earlier while saving the Red Sox money (and tax) later.
In this baseball economy you have to look at, say the 4 years after that at something like $40+ per year. So let's say 4/180 or something. So a total extension of 8 years 300 million makes sense. There is some room to go higher, but that is what you are looking at imo, more or less.