When an UFO is damaged (and crashes), its material value is accordingly lower, and all its fuel is lost.
The first subsystem that is damaged on a crashed UFO seems to be one of its engines.
It is advisable to store at least once a vessel from each class, in good shape, to know what and how many subsytems it holds, because you don't know when prompted to sell it (only its hull integrity in percents).
Here are some data (v2.4) if you want :
Maximum benefit is achieved with non damaged UFOs, and is to be added to the maximum derelict's selling price to nations to get the total income. I didn't include the materials' and fuel's selling, because you need them and they are of no value to non-PHALANX..
It is advisable to store at least once a vessel from each class, in good shape, to know what and how many subsytems it holds. You don't have this information when prompted to sell it.
Scout :
Derelict's selling price : 7 - 9 Kc
Dismantling cost : 4 Kc
Sold subsystems : 16 Kc
Max. benefit : 3 Kc
I didn't test much on scouts, because they are of low value. But as as soon as they have lost one subsystem (health < 68% ?), they are better sold to scrapers.
Fighter :
Derelict's selling price : 10 - 12 Kc
Dismantling cost : 4-5 Kc
Sold subsystems : 27 Kc
Max. benefit : 10 Kc
The main purpose of dismantling a fighter is to retrieve its weapon. The sooner the better, and it takes 1800 man.hours less than with the harvester. If you don't sell the weapon, then the benefit shrinks to nearly null. Early, you may dismantle fighters moderately to lightly damaged (when hull integrity is 56% - 75%), but know that a 50% damaged craft has only one engine to sell. As soon as you have harvesters to dismantle, always sell fighters, because for the same number of weapons, it takes two fighters, and time-wise, money-wise, fuel-wise, material wise, it doesn't match the one harvester. Moreover, you rarely get undamaged fighters... (early base and terror missions, I think).
Harvester :
Derelict's selling price : 16-20 Kc
Dismantling cost : 9 Kc
Sold subsystems : 61 Kc
Max. benefit : 32 Kc
Although I like Crashed Harvester missions, I refrain to take them down. Because they are my only energy source so far (and I don't know how much the advanced craft use per flight). And because they bear two weapons. As it takes time to dismantle (more than 4 days with 50 workers), and you have so many non damaged harvesters to retrieve, I didn't take time to try and dismantle damaged ones.