Plant cheri and sell the seeds.
These berries take 16h to grow; a watering at the start and at the 8h mark will be enough.
Harvest tools cost $350, and you need one per berry.
In Hoenn there are 33 spots + 72 at Mistralton + 84 at Abundant Shrine.
Say, you plant them all and the average yield is 4.5 per plant, you would get ~850 berries in 16h.
With cheri, the ratio of plain/very seeds is about 3:1 so in a batch you would obtain ~637 plain seeds and ~212 very seeds (it's actually higher since some produce 2 seeds).
RN GTL lists the cheapest seeds at $775 and $1,925 (plain, very), so you would obtain 637*775+212*1,925=$901,775
Discounting the seed costs (357*775=$276,675) and the tools costs (850*350=$297,500) you obtain a net gain of $327,600 for ~30min effective work each 16h.
This is for beginers as you will eventually find out that there are more effective methods.
Edit: You can also sell the berries and get a bit higher returns, but then you will have to actually re-buy your seeds and I feel the berry market is slower.