A city is planning to build a rock retaining wall, a monument, and a garden in a park. The table above shows four rock types that will be considered for use in the project. Also shown for each rock type is its weight per volume, in pounds per cubic foot (lb/ft$^{3}$), and the cost per pound, in dollars. The equation $0.03(120w)+0.18(180z)+3,385.80=7,576.20$ gives the total cost, in dollars, of the rocks used in the project in terms of the number of ft$^{3}$ of limestone, $w$, and the number of ft$^{3}$ of basalt, $z$. All four rock types are used in the project. Which of the following is the best interpretation of 3,385.80 in this context?