I really don't understand why that 15% MAG is so expensive, double the damn trust moogle
Long-read incoming, fair warning.
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The short answer is power creep relative to where the game is at the moment.
Think of it this way:
-- Consider a scale from 1 to 10. One is trivially easy to obtain, ten is incredibly difficult to obtain.
-- Consider that there are two scales in place in the game, a premium scale and a F2P scale. F2P scaled items are generally inferior to premium (think TM) items/abilities.
Items off the Mog King are technically "free", so they fall on the F2P scale. For a free item, the highest available materia for magic is +10%. (The Candy Basket from the Halloween event is an accessory, so it is not a valid comparison here.)
Therefore, as the new top tier free magic materia available in the game, it sits at the "10" point on the scale, and therefore is extremely expensive to obtain.
Over time, the scale will move forward and the materia will be priced cheaper in the future as MAG +15% becomes more common (again, only comparing against free items). Case in point is the HP +15% materia offered in this event. The first time it was offered, it required a ridiculous amount of drops and megacrysts to craft (a 10). The second time, it was offered for a high amount of event currency (a 9). Now, it is offered for a slightly above average amount of currency (perhaps a 7).
Are there premium items available that blow this out of the water? Of course. MAG +30 exists. Then again, that's a whole TM farm that needs to be completed for that.
Will MAG +15% be cheaper in the future? Undoubtedly.
The question then becomes "Is obtaining the best **free** magic materia in the game at this moment in time worth the significant premium tacked onto the price tag right now?"
That's a decision each person has to make on their own based on their own perception of value.
Some questions to ask yourself when valuing this materia:
-- Do I have access to Shantotto for MAG+30%
-- Do I have sufficient time to grind out her TM for a MAG +30%?
-- Do I use significant quantities of MAG +10% on my team now?
-- Would a MAG +15% be a desired upgrade for my best mage on my A-team?
-- As this is a limited materia to the event, would I be okay with skipping this for now and waiting likely at least another month for the next opportunity to obtain this materia?
-- Do I have sufficient Bonus units to reach the 70k karma needed for this materia (min required: 200-225% bonus with friend unit)
Hope this information helps!