sum += Convert.ToDouble(e.Row.Cells[4].Text)*0.7 + Convert.ToDouble(e.Row.Cells[5].Text)*0.2 + Convert.ToDouble(e.Row.Cells[6].Text)*0.1;

把这个分开，一个一个写！

double a = Convert.ToDouble(e.Row.Cells[4].Text  == "" ? 0 : e.Row.Cells[4].Text);
double b = Convert.ToDouble(e.Row.Cells[5].Text  == "" ? 0 : e.Row.Cells[5].Text);
double c = Convert.ToDouble(e.Row.Cells[6].Text  == "" ? 0 : e.Row.Cells[6].Text);

sum += a+b+c;

------解决方案--------------------

C# code
  //1.
          if (e.Row.Cells[4].Text.ToString() != "")
            { 
                //处理
            }
   //2.
//模板列的话这么处理
            TextBox txtPrice = e.Row.FindControl("tbPrice") as TextBox;


            if (txtPrice.Text.ToString() == "")
            {
                //处理
            }
            else
            { 
                //处理
            }