This is how I made gil for a big house.

[Saradina]

Yes it is, but I tried to do it this week to see if it still worked on Cerberus and lo' and behold I made 10 million gil.

Of course it's a bit RNG but still in the long run it's not, the thing which sells for more than 5 cheap materia VI are craftman + gatherer (about 500-600% profit. Savage aim and might about 100% profit and quickarm about 20-30% profit) those are 11 different materias and together you'll get 3860% profit in 11 materia then there's the "shit" materia which you will lose 80% of the value those are elemental and the stat stuffs so if my math is correct those are 9 different materia. So (3860/11) - (20x9) = 170% net gain.