Calculating percentage of left over cost displaying result a

Question

double number;
double number1;
double result;

double.TryParse(SelectedQuoteForEditing.JobPrice, out number);  double.TryParse(lblRate.Content.ToString().ToString(), out number1);

var left = Math.Round((number - number1) / number * 100, 2);

result = left;

ERGPerc.Content = result.ToString("P") + "%";

I am trying to show the percentage which is left after number1 has subtracted from number. The values I am calling contain a £ symbol and when running the code I get the result of "NaN" I guess this is to do with the £ sign as when removed code works. Anyone have any idea how I can fix this please ? I have been trying for hours with no luck :( Thanks in advance.