To find the cost of one shirt, we can use the cost and quantity values provided in the table.
First, let's look at the cost of shirts:
- For 6 shirts, the cost is $18, so the cost per shirt is \( \frac{18}{6} = 3 \).
- For 5 shirts, the cost is $15, so the cost per shirt is \( \frac{15}{5} = 3 \).
- For 4 shirts, the cost is $12, so the cost per shirt is \( \frac{12}{4} = 3 \).
- For 3 shirts, the cost is $9, so the cost per shirt is \( \frac{9}{3} = 3 \).
In all cases, the cost for one shirt is $3.
So, the answer is: $3