Answer
$Cost={$}0.063$
Work Step by Step
We can find the required cost as follows:
As , the threshold limit is $2\times 10^{-11}\frac{g}{L}$ ,therefore, we will convert the volume of air into liter
$V=5.0\times 10^7 ft^3\times\frac{(12)^3 in^3}{(1)^3ft^3}\times \frac{(2.54)^3cm^3}{(1)^3in^3}\times \frac{1L}{1000cm^3}=141.58\times 10^7L$
Now, can calculate the cost as given below
$Cost=141.58\times 10^7L\times \frac{2\times 10^{-11}g}{1L}\times \frac{$112}{50g}$
This simplifies to:
$Cost={$}0.063$
