Well, if it's loops they did, it'll cost them a D level service... ;-) Just taking the MTOM of 560t and MLM of 368t, at the most extreme that would have set them back about 190t of fuel, about 260k liters, that would be about EUR104k in fuel cost. Could be as little as EUR 60k or so also. I doubt the crew will tell....
A better estimate could be gleaned from how long they were dumping for. According to my FCOM the A380 can dump 150t of fuel per hour.