and i think i found a unique answer:

45 High Mythril Armor x 3 Town Some Dragoons Have All the Luck 977

This is the single most profitable leve, netting 2900 gil each time it is fully completed

now, if i were to do this leve and only this one, using loose math
10,000,000 gil needed, divided by 3000 gil, is 3333 turn ins, if turn ins double gil, cut turn ins in half 1500

let me go check this ingame, i need to make sure gil is doubled when turn ins happen