
Click cell B4, then cell C4.
Notice the formulas in the Formula Bar.
Both formulas use cell E8 in the Numbers Per Costume sheet.

Click cell C4, then place the cursor on the Fill Handle.

Drag it to cell M4.
The worksheet should look like this:
By employing an absolute reference—using the $ symbol to make each formula use the same cell containing the costume price—the formula stays accurate for all months.