Only conjoint flights with transfers at Beijing, Xiamen, Guangzhou or Inchon are available from Jakarta to Harbin. And taking the flight transferring at Beijing will cost less flight time. Here is one flight schedule for your reference: Jakarta-Beijing, 01:10-11:05, CA978, Air China; Beijing-Harbin, 12:55-14:35, CA1623, Air China

As for the cheap flight, it is hard to say since the airfare for the international flights varies on different date.