jmc

Let $x$ be the cost per square inch of gold paint, and let $y$ be the cost per cubic inch of concrete. Since a 1 inch cube has surface area 6 ${\text{in}}^2$ and volume 1 ${\text{in}}^3$ its total price will be $6x+y$ dollars. Similarly, a 2 inch cube has surface area 24 ${\text{in}}^2$ and volume 8 ${\text{in}}^3$, so its total price will be $24x+8y$ dollars. We are given that $$\begin{array}{rl}6x+y& =\$1.30\\ 24x+8y& =\$6.80\end{array}$$ Subtracting 4 times the first equation from the second gives 4y=\1.60$,so$y=\$0.40$. Hence 6x=\0.90$,so$x=\$0.15$. Since a 3 inch cube has surface area 54 ${\text{in}}^2$ and volume 27 ${\text{in}}^3$, its total price will be a total of 54(\0.15)+27(\0.40)=\boxed{\18.90}$.