eukar,
They probably save the SE you would make for every hunt and subtract the SE made from drakos on every fight.
For example, if you make an average of 2,000 SE per hunt and the drako gives you 1,000, the game will save 3,000 for each fight to give you at the end of the event.
And of course the 2,000 average considers SM, Trapping, Squeezing and Expert Hunt.
It is either this or something even better
They could also just save the number of hunts and calculate the payback at the end, when our levels will be higher.
Cruel.