BIG EDIT:
Edit 2:You can't gain MP, it will just cut the cost down to zero.
Spells you use Accession are not affected by the light arts cost reduction.
This is just a little numbergame in my head I came up with when we did some Draguas with a PT full of melees and a WHM spamming Curagas.
Orsn. Pantaln. +2 Rare Ex
[Legs] All Races
DEF:41 MND+7 Healing magic casting time -12% Converts 5% of "Cure" amount to MP "Divine Benison"+1 Set: Augments elemental resistance spells
LV 83 WHM
With enough HP missing on your PT members, these allow you to make your Curagas free. The effect can be enhanced with abilities like Divine Seal, Light Arts and Strategems, as well as of course maxing your cure potency.
The example value will be my Taru 99WHM/SCH with below average MND of 89+25, VIT 80, Cure Potency +50% and also low Healing Skill of 376.
Curaga:
MP cost: 60 / Arts 54
HP healing needed for 0MP cost: 1200 / 1080
Average HP healing per PT member: 200 / 180
Example Value: 144 HP
Pretty unreasonable without abilities.
Curaga II:
MP cost: 120 / Arts 108
HP healing needed for 0MP cost: 2400 / 2160
Average HP healing per PT member: 400 / 360
Example Value: 301 HP
Closer but no dice.
Curaga III:
MP cost: 180 / Arts 162
HP healing needed for 0MP cost: 3600 / 3240
Average HP healing per PT member: 600 / 540
Example Value: 606 HP
Here is the point of breaking even. But also the point where everyone has to have a big junk of HP missing.
Curaga IV:
MP cost: 260 / Arts 234
HP healing needed for 0MP cost: 5200 / 4680
Average HP healing per PT member: 867 / 780
Example Value: 1068 HP
Better have Galka DDs.
Curaga V:
MP cost: 380 / Arts 342
HP healing needed for 0MP cost: 7600 / 6840
Average HP healing per PT member: 1267 / 1140
Example Value: 1339 HP
Abyssea or Galka MNKs
As you can see it's possible from Curaga III on to nullify MP usage without any ability use. This would need everyone having lots of spread even damage from AoEs tho. I still like the number play, to see how cost efficient raw Curagas can get, even if you don't break exactly even.
Accession Cure IV:
MP cost: 88
HP healing needed for 0MP cost: 1760
Average HP healing per PT member: 293
Example Value: 610 HP (440 Healing Skill)
Accessioned Spells are VERY efficient. (If they works as expected with the WHM AF3+2 legs)
Feel free to add, correct or bash me on my little number game. I for one now have a little better understanding of my MP management on AoE heavy mobs.