More complex modeling would be needed to produce a reliable estimate of the cost of the tariffs, but the point is about the order of magnitude. These are not numbers that are enough to cause much damage to a $20 trillion economy, or to justify the $460 billion decline in the value of the stock market that took place between Thursday’s open and Friday midday.
This market drop makes more sense if you look at the president’s announcement not in terms of what it means for imported steel and aluminum, but rather what it says about the president himself.
I've heard from John Oliver and Trevor Noah that the tariffs reduce more jobs than they create, thus making it a blow to the job market. Seems economically significant to me — though I'm by no means an expert.