You have to select the format of "Number" and then you can select the currency symbol to display. The value of the field will only be the numeric value without the currency symbol. Having a field value with any symbol other than a number, coma, decimal, +, or - will cause the field to be considered a text string field and not a number field.
You have to use a non-numeric value for the start of a field name because the simplified field notation uses the non-numeric value to detect a field and not a numeric constant or operative symbol (+, -, *, /). You can also not call any function, use any field property or use any field method in the simplified field notation.
