$3.10

\begin{aligned}
\text { Total Cost } &=T \times C \\
&=\left[\begin{array}{ll}
4 & 3
\end{array}\right] \times\left[\begin{array}{l}
0.40 \\
0.50
\end{array}\right] \\
&=[4 \times 0.40+3 \times 0.50] \\
&=[1.60+1.50] \\
&=\$3.10
\end{aligned}