The trapezoid has parallel sides $24$ and $48$ with height $32$: $A = \frac{1}{2}(24 + 48)(32) = \frac{1}{2}(72)(32) = 1{,}152 \text{ ft}^2$

Gallons needed: $\dfrac{1{,}152}{300} = 3.84 \Rightarrow 4$ gallons (sold by the full gallon). $\text{Cost} = 4 \times \$16.50 = \$66.00$