The expectation and the variance for a Poisson random variable are the same. But the poisson-distributed random variables here are the number of calls, not the cost. Both the expectation and variance for the number of calls are dimensionless.
If you multiply a random variable with a constant factor, the variance gets scaled with the square of this factor. Therefore, if u is the number of calls during daytime and U the costs, Var(U)=(50$)^2 * var(u) = (50$)^2 * E(u) = (50$) * E(U) = 157500$^2
Similarly, Var(V)=60$*E(V)=216000$^2 and Var(U+V)=373500$^2.