I believe Betts' current contract (arbitration) is for roughly $20 Million per year and that contract expires after the 2020 season. He'll likely get Trout/Harper/Mancado salary numbers when he hits free agency, i.e. +$30 Million per year.
Henry, the Red Sox owner, has advised that 1) the Red Sox want to get below the MLB luxury tax number for 2020 (projected to be $208 Million in 2020 whereas it was $206 Million for 2019 and the Sox [one of 2 teams above the threshold, the other being the nationals] were at $238 Million [via ESPN]). he has already admitted that re-signing both Betts and JD Martinez (will likely opt for free agency 2020) will be very difficult.
Excluding Martinez's 2020 salary of $23.75 Million due to above noted pending free agency, the Sox already have $112 Million committed to just 5 players - Price, Sale, Bogaerts, Pedoria (ouch), and Evoldi. Subtract Pedroia's $13 Million if he retires (likely) and add in say $23.5 Million for another Bett's arbitration year plus $10 Million for an actual closer (desperation) and the Sox look to have $132.5 Million committed to 6 players. So, under a hypothetical 'blank slate roster beyond those 6, the Sox have to spend less than $2.15 Million on average for the remaining 35 active roster slots on the 40-man roster in 2020. Good luck! Plus, their farm system was trashed to win the title last year.
Thus, the talk that the Sox may trade Betts to A) avoid his 2021 +$30 million salary hit and B) he should yield more prospects for the farm system with 1 more year under arbitration than Martinez who can (will) hit free agency in 2020.