For the 3 current contracts named that are on the 40-man, we don't know yet.
Acquiring top tier players who have any choice in the matter will always seem "not cost effective" because the collective bargaining agreement makes it that way. Multiple teams will bid up anyone perceived as above average, creating the classic s-shaped curve (here, money spent on the horizontal axis, value on the vertical).
I'll go along with others that Donaldson's contract was bad for the Twins, because it cost them significant resources in trade to simply get out from under it. Buxton's contract has simply cost money, so far; the terms of the contract were meant to protect against injury but "oh, I can only DH today" for weeks on end proved to be a loophole, and if that persists through the life of the contract I'll change my view. Correa and Vazquez, I foresee them delivering more value than they have so far, so the jury's out.
Dobnak is looking like a failure of player evaluation, with the finger injury being merely what triggered it. It bothers me more than it should, because the contract is the least consequential of those named.