Update: I made a silly math mistake. That's $9 profit per shirt. So if you make 100 shirts but only sell 50 and burn the rest, that's $450 profit. You make $4.50 per shirt manufactured.

Stated another way: you can total up the manufacturing cost of the shirts you destroyed ($50) and distributed evenly among the ones you sold (50/50=$1 each) and just add that to the cost of each shirt you sell when calculating profit. Same result.

If you throw some plastics into a coal fired power plant it is almost the same as if you would burn oil.