Not much - they would likely get about $10 per plaque from a scrap metal dealer. But the scum who do this are often doing it for money to buy drugs and they will try anything.
And the best chance for catching them will be a scrap metal dealer calling the police - there are plenty who will, but others are just as bad as the thieves themselves. It’s an industry that seems to attract some ‘no questions asked’ types.
however it comes about, I hope that the thieves will get thiers