Error formating number

public double getValorPropinaPagoMesYear(string id_c, int mes, int year)

{

string sql = "select sum(valPago) as n from estudanteMensalidade where ficha_id='" + id_c + "' and ano=" + (year - 2000).ToString() + " and mes=" + mes.ToString();

using (SqlConnection cn = new SqlConnection(con.stringConexaoRH()))

{

try

{

cn.Open();

using (SqlCommand cm = new SqlCommand(sql, cn))

{

using (SqlDataReader rd = cm.ExecuteReader())

{

if (rd.Read())

{

try

{

return Convert.ToDouble(string.Format("{0:0.00}", rd[0]));

}

catch (Exception ex)

{

return 0;

}

}

else

{

return 0;

}

}

}

}

catch (Exception ex)

{

MessageBox.Show(ex.ToString());

}

}

}

 

Please Help!