This could be extremely valuable, or complete garbage. Hard to tell.
The Common:Uncommon:Rare ratio is 22:5:1. The chance of a drop varies depending on the rank of the enemy defeated:
Each tier of salvage corresponds to the level range of Inventions which the salvage from that tier will be used in.
Which tier of salvage dropped is determined entirely by the level of the NPC you defeat. The tiers work like this:
For more information refer to InfamousBrad's Invention Salvage and You!(Version 2)