Theres a chance that it could not be random at all but be a standard amount given to all party (eventually multi party) groups involved in the event if its a success. I would prefer this, altered to have a FFXIV spin of course, thats just the barebones idea.

Say there is a RA/EX currency called bologna (just to be silly) and you need 50,000 of it as part of your quest for a relic weapon. There could be two events, 1 a daily that a single party does and if you win you get 30 bologna each. and then there could be a multi party event that occurs 2 times a week max that is much harder/longer, and upon success you could get 70 bologna each. possible extra goals performed mid event could boost it by 5 and 10 respectfully. Bologna goes into your currency item pouch and would be like marks so that you can't give to other people.