Input string was not in a correct format

I have qi1 decimal field 
coming error in line
txtCostPrice = Convert.ToInt32(((TextBox)GridView2.Rows[i].FindControl("txtcostprice")).Text);

my code
public void savedata()
    {
        con = new SqlConnection(dbcon);
        con.Open();
        int txtCostPrice = 0;
        //int txtSellPrice = 0;
        int Primarykey = 0;
        string ProductName = "";
        DataTable dt = new DataTable();
        dt.Columns.Add("idsd", typeof(int));
        dt.Columns.Add("aname", typeof(string));
        dt.Columns.Add("qi1", typeof(int));
        //dt.Columns.Add("qi2", typeof(int));
        da = new SqlDataAdapter("Select * from studentF1", con);
        SqlCommandBuilder cmb = new SqlCommandBuilder(da);
        da.MissingSchemaAction = MissingSchemaAction.AddWithKey;
        ds = new DataSet();
        da.Fill(ds, "studentF1");
        for (int i = 0; i < GridView2.Rows.Count; i++)
        {
            Primarykey = Convert.ToInt32(GridView2.DataKeys[i].Value);
            ProductName = ((Label)GridView2.Rows[i].FindControl("lblproductname")).Text;
            //txtCostPrice = ((TextBox)GridView2.Rows[i].FindControl("txtcostprice")).Text;
            //txtSellPrice = ((TextBox)GridView2.Rows[i].FindControl("txtsellprice")).Text;
            //txtCostPrice = Convert.ToDecimal("txtcostprice");
            txtCostPrice = Convert.ToInt32(((TextBox)GridView2.Rows[i].FindControl("txtcostprice")).Text);
            //txtSellPrice = Convert.ToInt32(((TextBox)GridView2.Rows[i].FindControl("txtsellprice")).Text);

            DataRow dr = ds.Tables["studentF1"].Rows[i];
            if (txtCostPrice.Equals(string.Empty))
                dr["qi1"] = 0;
            else
                dr["qi1"] = txtCostPrice;
            //if (txtSellPrice.Equals(string.Empty))
            //    dr["qi2"] = 0;
            //else
            //    dr["qi2"] = txtSellPrice;
            
            
            da.Update(ds, "studentF1");
        }
        con.Close();
        bind();
    }