If you make Rp. 1,800,000 a year living in Indonesia, you will be taxed Rp. 158,400. That means that your net pay will be Rp. 1,641,600 per year, or Rp. 136,800 per month. Your average tax rate is 8.8% and your marginal tax rate is 8.8%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of Rp. 100 in your salary will be taxed Rp. 8.80, hence, your net pay will only increase by Rp. 91.20.
A Rp. 1,000 bonus will generate an extra Rp. 912 of net incomes. A Rp. 5,000 bonus will generate an extra Rp. 4,560 of net incomes.