nedash says:
protected void Imgorder_Click(object sender, EventArgs e)
{
    ImageButton ibtn = sender as ImageButton;
    CheckBox ChS1 = ibtn.NamingContainer.FindControl("ChS1") as CheckBox;
    CheckBox ChS2 = ibtn.NamingContainer.FindControl("ChS2") as CheckBox;
    CheckBox ChS3 = ibtn.NamingContainer.FindControl("ChS3") as CheckBox;
    int id = Convert.ToInt32((sender as ImageButton).CommandArgument);
    DataTable dtFiles = GetFilmInfo(id);
    string ImageName = dtFiles.Rows[0][22].ToString();
    string Se1P = dtFiles.Rows[0][24].ToString();
    string Se2P = dtFiles.Rows[0][28].ToString();
    string Se3P = dtFiles.Rows[0][32].ToString();
    string Se1 = dtFiles.Rows[0][23].ToString();
    string Se2 = dtFiles.Rows[0][27].ToString();
    string Se3 = dtFiles.Rows[0][31].ToString();
    string Code = dtFiles.Rows[0][4].ToString();
    string Daste = dtFiles.Rows[0][1].ToString();
    DataTable dt = new DataTable();
    if (Session["Order"] != null)
    {
        dt = Session["Order"] as DataTable;
    }
    else
    {
        dt.Columns.AddRange(new DataColumn[] { new DataColumn("Id",typeof(int)),new DataColumn("ImageName",typeof(string)),new DataColumn("SeP",typeof(decimal)),
                       new DataColumn("code",typeof(string)),new DataColumn("Daste",typeof(string)),new DataColumn("Se",typeof(string))});
 
    }
    if (ChS1.Checked || ChS2.Checked || ChS3.Checked)
    {
 
        if (ChS1.Checked)
        {
            dt.Rows.Add(dt.Rows.Count + 1, ImageName, Se1P, Code, Daste,Se1);
        }
        if (ChS2.Checked)
        {
            dt.Rows.Add(dt.Rows.Count + 1, ImageName, Se2P, Code, Daste, Se2);
        }
        if (ChS3.Checked)
        {
            dt.Rows.Add(dt.Rows.Count + 1, ImageName, Se3P, Code, Daste, Se3);
        }
        Session["Order"] = dt;
  
 replace above code with below and make sure you convert SeP1,SeP2,SeP3 to decimal so that you wont get conversion error
protected void Imgorder_Click(object sender, EventArgs e)
{
    ImageButton ibtn = sender as ImageButton;
    CheckBox ChS1 = ibtn.NamingContainer.FindControl("ChS1") as CheckBox;
    CheckBox ChS2 = ibtn.NamingContainer.FindControl("ChS2") as CheckBox;
    CheckBox ChS3 = ibtn.NamingContainer.FindControl("ChS3") as CheckBox;
    int id = Convert.ToInt32((sender as ImageButton).CommandArgument);
    DataTable dtFiles = GetFilmInfo(id);
    string ImageName = "Breaking Dawn";//dtFiles.Rows[0][22].ToString();
    string Se1 = "1"; // your number of season;
    string Se2 = "2"; // your number of season;
    string Se3 = "3"; // your number of season;
    string Code = "1000";//dtFiles.Rows[0][4].ToString();
    string Daste = "Film"; //dtFiles.Rows[0][1].ToString();
    decimal SeP1 = 2000; //dtFiles.Rows[0][24].ToString();
    decimal SeP2 = 2000; // dtFiles.Rows[0][28].ToString();
    decimal SeP3 = 2000; //dtFiles.Rows[0][32].ToString();
    decimal totalPrice = 0;
    string totalSeasonsSelected = string.Empty;
    DataTable dt = new DataTable();
    if (Session["Order"] != null)
    {
        dt = Session["Order"] as DataTable;
    }
    else
    {
        dt.Columns.AddRange(new DataColumn[] { new DataColumn("Id",typeof(int)),new DataColumn("ImageName",typeof(string)),new DataColumn("SeP",typeof(decimal)),
                    new DataColumn("code",typeof(string)),new DataColumn("Daste",typeof(string)),new DataColumn("Se",typeof(string))});
    }
    if (ChS1.Checked || ChS2.Checked || ChS3.Checked)
    {
        if (ChS1.Checked)
        {
            if (!string.IsNullOrEmpty(Se1) && SeP1 != 0)
            {
                totalSeasonsSelected += Se1 + ",";
                totalPrice = totalPrice + SeP1;
            }
        }
        if (ChS2.Checked)
        {
            if (!string.IsNullOrEmpty(Se2) && !string.IsNullOrEmpty(totalSeasonsSelected) && SeP2 != 0)
            {
                totalSeasonsSelected += Se2 + ",";
                totalPrice = totalPrice + SeP2;
            }
            else
            {
                totalSeasonsSelected += Se2 + ",";
                totalPrice = totalPrice + SeP2;
            }
        }
        if (ChS3.Checked)
        {
            if (!string.IsNullOrEmpty(Se3) && !string.IsNullOrEmpty(totalSeasonsSelected) && SeP3 != 0)
            {
                totalSeasonsSelected += Se3 + ",";
                totalPrice = totalPrice + SeP3;
            }
            else
            {
                totalSeasonsSelected += Se3 + ",";
                totalPrice = totalPrice + SeP3;
            }
        }
        dt.Rows.Add(dt.Rows.Count + 1, ImageName, totalPrice, Code, Daste, totalSeasonsSelected.Substring(0, totalSeasonsSelected.Length - 1));
        Session["Order"] = dt;
    }
}