Bello:
2024 (25) - pre-arb - something like league min ($740k)
2025 (26) - pre-arb - something like league min ($760k)
2026 (27) - arb 1 - something like $8m
2027 (28) - arb 2 - something like $15m
2028 (29) - arb 3 - something like $20m
2029 (30) - free agent
So for 2024-25, Bello will make $1.5 million combined. Then from 2026-28, and I'm guessing here, he might make something like $43m combined ($14.3m average). That means that over the next five seasons, he probably would make about $44.5 million - let's round up to $45 million, or an average of $9m per season.
Then he hits the open market, heading into his age 31 season. Yes, if he's great at that point, he'll cost a fortune, but he also would be on the wrong side of 30. Probably some good years left in him, but also it's the old "do we give a long, lucrative contract to a 31-year old pitcher" question. And at that point he'd be looking at, what, like $50m a season? Maybe more, given how fast contracts are rising?
So a deal where they buy him out through 2029 only buys out one year of free agency, and if they give him on average $20 million a year from 2024-2029, that's five seasons (2024-2028) x $20m = $100m, which would be more than twice what it will cost him just letting him go his normal path over those years as outlined above. But that 2029 season would still only cost $20m, which would be far less than what a free agent season would cost.
The point is, it's worth it if the Sox can maybe sign him now to a long-term deal at between $20-25m through 2032 or something like that. But if they only sign him through 2029, it's probably not worth it financially, because they'd be paying a hell of a lot more for 2024-28 than normal, PLUS they'd be taking on enormous additional risk given how pitchers break down these days.