How much would taxi cost from guangzhou airport to skyline plaza hotel at 12am?

If by taxi from the airport, it costs CNY 90 to 100 at that time.
The hotel doesn't provide free pick-up service. And according to the arrival time, there is no metro at airport.
You can try to catch airport shuttle bus line 5, get off at Haizhu Square first. The service hours of this bus route are from 06:45 to 01:00 next day with the frequency of 15 mins. After arriving, take a cab to the hotel. The cab fare costs about CNY 10.
P.S: Your hotel is quite close to Shiergong metro station so that you can easily tour around by metro.