MArcomage

Common? You sure?
In that case it needs a cap at 5-6 or something.

That or lose the Brigand keyword. Either works.

By the wording, I assume that this works with all types of discard. If it only works on standard discard, there would be no need to specify the card with highest cost.

So if the opponent discards his hand with Auxilia and one of those discarded is All-elemental attack (cost: 18/18/18), this card will most likely grant 6 stock? That's still a rather amazing amount of gain for something the opponent may be unable to avoid allowing.

This would work so effortlessly well with a Brigand deck that I, too, think it should lack the keyword. It's just too easy. If not that, make it Uncommon.

I'd be fine with it working in almost any situation, my suggestion is to have a card that gives you resources back from your opponent's discard (similar to Farm).