A small retailer is considering various monthly plans for purchasing electricity. They can pre$-$purchase kWhs at the cost of $4\mathrm{.}5$ cents per kWh$.$ If they don$$t use all of the kWhs they purchase, then there is no refund for the unused electricity. If they need more kWhs than they purchase, the extra kWhs are purchased at the rate of $6$ cents per kWh$.$ Their forecast for kWh usage per month is normally distributed with a mean of $6000$ and a standard deviation of $2500.$

What is the cost of under$-$estimation?$$