If you make 1,800,000 SAR a year living in Saudi Arabia, you will be taxed 63,000 SAR. That means that your net pay will be 1,737,000 SAR per year, or 144,750 SAR per month. Your average tax rate is 3.5% and your marginal tax rate is 1.0%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of 100 SAR in your salary will be taxed 1 SAR, hence, your net pay will only increase by 99 SAR.
A 1,000 SAR bonus will generate an extra 990 SAR of net incomes. A 5,000 SAR bonus will generate an extra 4,950 SAR of net incomes.