I've had decent luck with metal ghosts (~8000 karma when ghosts spawn with a friend elza + my tilith), so I'm almost done with this event. I picked up all the tickets, quartz, 6* mats, cactuars, consumables, screamroots, and both weapons. The only thing left worth buying is the trust moogle and the 2x destroyer materia.
Going to skip the materia I think, because 10% hp, 5% atk, 5% mag probably isn't worth equipping over a 10% mag or 10% atk, and we're getting 10% atk in ~2 weeks.
I think I will farm the moogle, especially since I used 3 recently to finish dualcast, so I'm down to 3 moogles remaining. With 1-2 more ghost spawns, it's worth the energy to farm it. Even if I go the rest of the 35k karma without ghosts, using 2 BF units it's ~500 karma per run, so that's 70 runs, or 910 energy. Bad ratio then, but I doubt I go 70 runs without a single ghost spawn.
It's not half bad exp either per run. I was using a 5 man team with tilith in it, and she's already level 29 or something. Think I'm going to cut it down to the bare minimum to clear it (likely just Cecil + CoD + Tilith w/ friend) so she gets even more xp, or maybe toss in some other units to level too, like my lvl 58 BCLid.