Found the costs, I think.

Five steps, these need to be repeated PER SOUL.

Step 1:

30 SR Books. Did you want to use your black books for SR Hime? TOO BAD.
10 Demonic Cores. What's that, you're not a player who isn't drowning in Demonic Cores? TOO BAD, a single Advent gives you 9 Demonic Cores and you need 10 to FLB the weapons. And now you also need 10 PER SOUL. And this is assuming that Nutaku won't be cutting Advents short, we had one Advent that was lik four days since they needed to get back on DMM schedule...
50 story mode drops of each type. Have fun farming all the zones for materials. Fun fact, the first material will the most annoying one, since there's no 8-9 mob place to farm them in. Very cheap per AP, but dem loading screens.

Step 2:

20 Gold books, won't be an issue for free to play hello my stockpiles of 68, despite spending stupid amounts on this SSR Hime guaranteeds, but this step is a whale killer.
3000 Eidolon Orbs. Did you want Kaisers? TOO BAD.
150 T3 Potatoes aren't a problem by themselves, but did you want to exchange any for T4 Stars? TOO BAD.
150 Fangs of each element won't be a problem for Veterans thanks to them dropping in Gem Gacha and T4 Star hunts, but even for me I can only have enough to unlock two Souls.

Step 3:

1 Plat book. That book is from Phantom Olympia raid I believe, so you can basically think of it as 100 Magnas. Not sure.
5 Ori. One year into the future, this will be NOTHING for veterans. I'm surprised it isn't 50.
150 T4 Stars? OH YEAH, did you like having FLB Union weapons? TOO BAD, that's not allowed!
250 T2 Cores of each element is a giant bitchslap for new people, but also drops from Gem Gacha now. Still, that is a lot of them, and even I only have enough to unlock ~4 Souls atm.

Step 4:

1 Plat book (again). Another 100 Magnas.
10 Ori (again). Still not a problem.
1 Dragon Eye. Not a problem for anyone.
150 Regalia of each element. Oh yeah, did you think you could save up T2 Cores? TOO BAD, better get farming those Ults!

Step 5: (repeatable)

To my understanding, this requires a Soul weapon at level 125 and slv20. You get a FLB'd Soul weapon from this step. You can repeat this for as many elements as you want.
3 Plat books. That's 300 Magnas per FLB Soul weapon.
500 Soul Points. Well, not really an issue I'd think. One year into the future, you'll have all the MP you want. Wait, this is SP, not HSP right...?
1 Dragon Eye. Doubt it'll be a problem for anyone.
20 SR Books, 10 SSR Books. Oh yeah, get fucked.
150 T4 Stars. Did you think that step 3 was bad? HAHAHAHAHA.
150 Fangs (of respective element).
50 Magnas (of respective element).
50 Olympia Mats (of respective element).