To find out how much the stadium paid for the T-shirts, we can multiply the number of T-shirts by the cost of each T-shirt.
The number of T-shirts given away is 100, and the cost of each T-shirt is $15.
\[ \text{Total Cost} = \text{Number of T-shirts} \times \text{Cost per T-shirt} = 100 \times 15 = 1500 \]
Therefore, the stadium paid $1,500 for the T-shirts.