You can buy low and sell high.

I have botanist level 50 (though its not really necessary) I usually gather 99 flax seed and sell it for 100 + gil each. Thats close 10k profit

Or you can farm apkallu eggs and night milk on costa del sol or fleece in coerthas. I can usually make 100k a week.

Or you can invest in leveling a crafter, but it'll be VERY expensive, so choose the right 1.