A Profit Synth?!

Incredibly, there is actually a profit synthesis for skill up, and it's in Goldsmithing, of all things.

Goldsmithing: Lv.13.2 -> Lv.14.4

Desynthesis: Brass Ingot (Lv.15)
Lightning Crystal, Brass Ring (from NPC)

Consumed:
Lightning Cyrstals x118
Brass Ring x 87

Results: Brass Ingots x59

Gross Expense: 16,008 Gil
(Adjusted Expense: ~36,000 Gil)
Expected Recoup: 44,000 (selling only 4 stacks)

The Lightning Crystals were from my stores and donation from Joncat; the total amount used is worth 20k on Jeuno Auction House currently, so inflated the cost from 16k to 36k. Still, even given that calculation, this skill up session came out quite nicely at 8k ahead, and even let me squirrel away 11 Brass Ingots at the end.