Kind of sounds like a schizo question but I was always under the impression that reclaiming a live structure utilized the same logic as building a structure. Namely, the speed is determined by the buildtime divided by the total buildpower.
However, reclaiming a live building is instead reliant on the total energy to build the structure divided by the total buildpower. Why? I don't understand why it is suddenly necessary to swap the numerator. For the sake of intuitiveness it should just be based on buildtime; I can't really imagine a situation where this destroys game balance.
Example: a T3 pgen has 6800 buildtime and a T3 mass fab has 5000 buildtime. However, a T3 engie will reclaim 2 T3 pgens in the time it takes to reclaim 1 T3 mass fab. This is because a T3 pgen costs 58,000 energy and a T3 mass fab costs 120,000 energy.