If you make 1,200,000 Kč a year living in Czech Republic, you will be taxed 281,160 Kč. That means that your net pay will be 918,840 Kč per year, or 76,570 Kč per month. Your average tax rate is 23.4% and your marginal tax rate is 26.0%. This marginal tax rate means that your immediate additional income will be taxed at this rate. For instance, an increase of 100 Kč in your salary will be taxed 26 Kč, hence, your net pay will only increase by 74 Kč.
A 1,000 Kč bonus will generate an extra 740 Kč of net incomes. A 5,000 Kč bonus will generate an extra 3,700 Kč of net incomes.