When working with dollar sign expansions, put your expression into a Table Chart without a label. When the chart is rendered, the label will be your expression with the dollar sign expansion evaluated. That can be very helpful when working with complicated expressions.
Did you use an equals sign instead of that pound symbol?
Can you get the expression to work if you hardcode the answer?
If that doesn't work, you may need quotes around the 7 (if that is a string). Check the values of your month field, are you sure they are numerals?
Did you put it into a unlabelled expression? If your dollar sign expansion null or your entire expression?
You may need: