Campbells contract cost the team 3.9 mil in cap space this year due to being in the minors. It would be the same next year. That's his hit.
Buying him out means you save 3.9 this year out of 5. In reality you are paying 1.1m out of 3.9m. Hence the 2.8 in saving.
It gets worse in year 2 and 3 where the "savings" are only 1.6M and then 1.3M.
Now if they bit the bullet and paid the cost of trading him, they would save a full 3.9M over three years and another 1.5 mil for the three years after that.
It's not as rosy as some may think.
Nothing wrong with your premises. You are correct that when you compare a buyout to burying that the savings for the first 3 years are 2.8, 1.6 and 1.3.
You are also right that trading him would have saved them even more money. This isn't rocket science. It would have been nice to save an additional $10M from trading him.
The problem is that trading him would have cost an arm and a leg outright. If the cost to eat $5M is a 1st round pick, you would have traded 3 1st round picks. So that's not an option.
So I ask again, what would you do? Do you retain? How much? 50%? 25%? What do you give up?
Slice it however you want you need to give up something to make Campbell go away. The least painful option was to keep our assets and have a small % of the cap for the next 6 years.