What determines if a Mech is available as salvage?

It appears that the way to claim salvage is not linked to the recover-ability of the mech itself (I.e. how much damage you do to it), but rather in how much you invest into the shares at the beginning of the mission, and the size of the mech(s) you are attempting to salvage.

I don't have a precise graph at this point as to the ratio of shares:weight ratio, is, but this video explains that the maximum amount of salvage shares is 18, which covers a 60-65 tonne mech.

There appears to be a few factors that go into accessing shares, such as reputation with the employer of the mission, and your own mech reputation.