The target artifact card must have a lesser mana value than the artifact that caused Scrap Trawler's ability to trigger by being put into a graveyard. Use the artifact's mana value as it last existed on the battlefield to determine what may be returned.

If an artifact is a copy of another artifact with greater mana value, such as Sculpting Steel copying an artifact with mana value 4, Scrap Trawler's ability can target that artifact card in your graveyard when that artifact is put into your graveyard.

While on the battlefield or in a graveyard, in an object's mana cost is 0.

If Scrap Trawler and another artifact you control are put into a graveyard at the same time, Scrap Trawler's ability triggers for each of them.