Don't know what server you're playing on but I'm gonna be stupid and tell you a secret of mine.

I only make money through buying cheap and reselling things on the market. I mostly focus on items which sell very fluently and are not 100% stable when it comes to prices. On my server it's mostly materia V and VI. The thing is don't only go for big profits. I buy 20-50 materia V every day for 9 to 11k and resell them on the same day (or the next day in the morning) for 16 to 18k. Sometimes the market goes crazy for whatever reason and the price goes up to 28to 30k.

This is no jackpot but for me it is a very steady way of income.