It's more than likely a bit higher than people realize because they tested the waters with Fantasias as people were begging SE to let them buy it, then let them buy unlimited. Now some people are in desperate need of INventory (and really, what game DOESNT have free inventory increases at some point?) but we're given the excuses "the servers will crash" but they can sell us retainers no problem. So while it's not accessing the same memory, it is also part of the "every 5 seconds auto-save" (or 3?) that they say the game uses.
So they can't offer us the retainers (and thus more inventory space for people) for free, but can easily sell it to us. I know more than a handful of people got a 3rd+ retainer off the bat and people who tend to maximize profits usually employ 4+ retainers alone. The cash shop is extremely lucrative to SE which is why they continue to add to it, because SE has proven they'll easily cut their losses on something that isn't doing anything for them, e.g most mobile and their cloud services the past few months.
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