How to update text box value when the date is changed

I have a registration page

NoOfmonths(drop down list values are 2 3 4....)
StartDate
End Date(automatically comes based on selection of above no of months if number of months 3 then end date=startdate+3 months)
Payment is 5000

i have saved these details in database and binded to grid view

whenever the start date is crossed one month i want to update payment value to double
like that i want to update payment for 3 months(example for 3 months 5000+5000+5000=15000 )

this is my code

SqlConnection con = new SqlConnection(@"server=SYS8\SQLSERVER2008DE;database=Naresh;Integrated Security=true");
protected void Page_Load(object sender, EventArgs e)
{
SqlCommand cmd = new SqlCommand("select * from ValueUpdate", con);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();
}
protected void txtStartDate_TextChanged(object sender, EventArgs e)
{
string inputString = txtStartDate.Text;
DateTime dt = DateTime.ParseExact(inputString, "MM/dd/yyyy", CultureInfo.InvariantCulture);
dt = dt.AddMonths(Convert.ToInt32(ddlmonths.SelectedItem.ToString()));
txtEndDate.Text = dt.ToString("MM/dd/yyyy");

}
protected void Button1_Click(object sender, EventArgs e)
{
con.Open();

SqlCommand cmd = new SqlCommand("Insert into ValueUpdate (NoOfMonths,StartDate,EndDate,Payment) values('" + ddlmonths.SelectedItem + "','" + txtStartDate.Text + "','" + txtEndDate.Text + "','" + txtMonthlyPay.Text + "')", con);

cmd.ExecuteNonQuery();