At midnight about 12:30 am if we are 4 in the family with 2 luggage in Beijing. How much is the NET taxi fare ?

Would you tell me the departure place and destination? Then I ca tell you an approximate price. Anyway, here is the charge standard for your reference:
CNY13 for the first 3 kilometers (1.86 miles);
From 23:00 to 5:00, the charge per kilometer rises 20% i.e. to CNY 2.76.
If a single journey is longer than 15 kilometers (9.32 miles), the empty run fare will be charged i.e. the price per kilometer rises 50% to CNY 3.45;
CNY 1 for one ride for fuel surcharge.