How much fuel does the A380 use for taxiing before takeoff?
Fuel burn
To be meaningful it's best to compare it. It is approximately 4 times what an A320 needs. So if both planes are fully booked, then in theory each passenger should pay the same for the taxi fuel -- it would become an issue though when the A380 has a low load factor.

Taxi Fuel is 1 170 lb for an average taxi duration of 12 min.

While for 12 minutes an A320 needs 300 lb.
Cost
As of 20 April 2018, it would cost Emirates -- the largest A380 operator -- about \$360 for a 12 min taxi in Dubai (less than a dollar per passenger).
But Dubai is a busy airport and as such the taxi duration can exceed 12 minutes. For example Emirates 203 from a few days ago had a taxi time of 30 minutes. Operating a large airplane at a busy airport has its costs.

References:

Airbus FCOMs for the A320 and A380
IATA fuel monitor (jet fuel cost varies by region).