I'm not sure if this was intentional buuuuuut...
Basically around 3 million mesos worth of tradeable, sellable drops from Black Bean.
Which is basically a level 70 "boss" that can be wiped out with even the weakest of mules... with five runs per mule allowed.
So if you have say... twenty mules that are at least level 120... you can rack up over 50 million mesos a day in no time flat... I mean, seriously, it takes all of like 3 seconds to kill this thing. And your main can likely kill it in one hit.
This goes on till the 23rd too, so you can basically rack up around a billion mesos this month with twenty low level mules. ಠ_ಠ
...well, presuming they don't nerf it that is. They may have intended to make those drops untradeable.
That's just using a spider though, using greed pendants too you probably get more, plus that's not counting the money bags he drops, which I think is like at least 50k per run, so a quarter mill for five runs.
This is mostly beneficial if you don't have a "god level" character. I mean, if you do, then obviously you could do things like CRA and such (although a lot of those equips have become nearly worthless) but for most players in general this is an extremely fast/easy way to make a whole horde of money. Potentially even more profitable than Commerci voyages (although you have to be like level 160 for that). This you can do with sub level 100 players. Oh and it gives a bunch of experience too! Well, a "bunch" if you're below level 150 anyway.