If the shirt costs $10 in labor, and the pre-tariff wholesale price is also $10, how does the manufacturer make a profit? Surely the wholesale price would at least include some markup for profit? So like $10 labor + $3 markup (per employee-day) + $10 tariff ($23) contrasted with $15 labor + $3 markup + $4 tariff ($22), in your scenario.
But now you see that the low wage manufacturer has a third option, $10 labor + $1 markup + 10 tariff ($21), which would maintain their competitive advantage and in this scenario only cut their daily per employee profit by $2, as opposed to the $5 hit they would suffer by raising their employee wages to $15 day from $10.
What I'm saying is set a minimum wage on imports. Set a global minimum wage. Anyone importing something from somewhere that doesn't meet that minimum wage would have to pay a tariff greater than the amount saved with low wages.
The goal is to increase competition and improve fairness between locations. You wouldn't want to do it all at once at first, rather a gradual increase. You wouldn't want to distort the local economy too much so don't insist somewhere pay 10x the median local wage. Lots of things you would do which are more complex with an eye for fairness and competitiveness and definitely not trying to raise everyone by force to American levels of wages, but always measured amounts of
pressure.
But now you see that the low wage manufacturer has a third option, $10 labor + $1 markup + 10 tariff ($21), which would maintain their competitive advantage and in this scenario only cut their daily per employee profit by $2, as opposed to the $5 hit they would suffer by raising their employee wages to $15 day from $10.