1. Empty your inventory

If you want to gain as much profit as possible, empty your inventory!
Trust me, you'll need the space.

2. Gather green herbs

There are 7 green herbs in Spring.
2 are in the forest to the left of your farm.
5 are in the mountain to the north of your farm.

Gather them, don't use them until you absolutely need it.

Now, you're all set.

1. Get down to Floor 25.

!!!!IT IS OKAY IF YOU BECOME EXHAUSTED!!!
However, you !!!MUST NOT USE YOUR GREEN HERBS!!!

In fact, you will have a far easier time getting down to Floor 25 if you become exhausted.
It will take much, much longer for you to get down that deep as a fresh character if you try to conserve stamina.

2. Locate an automaton.
Automatons are the little mechanical, cog-powered mobs that walk around dropping bombs.

These bombs are the one of the keys to this method of acquiring cash.

3. If you are exhausted, eat a green herb.

This is the second key.
There is a mechanic in place that allows you to continue gaining experience, AND DROPS, once you eat something and put your stamina above 0.

4. Wait for the automatons to drop a bomb, and throw it at a wall.

When the bombs detonate, they do so in a 3x3 area centered at the bomb tile. This will damage you, the automaton itself, other enemies, and break the cave walls.
This takes NO STAMINA.
As long as your stamina is above 0, you will get drops.
I believe it is affected by your Mining level, however I am unsure if bomb-destroyed walls give you Mining experience.

5. Continue mining with bombs, and make sure that automaton does not die.

You can do so until the entire level is mined or until your inventory is full.
If you inventory is somehow not full and the level is mined, just go up and down the ladder until you find another automaton.

6. Return to the surface, drop your ore into the shipping container, and go back again.

Maximize your profits.
A full stack of silver is worth more than a full stack of zircon, even though one zircon is worth more than one silver.
At floor 25, you're looking for full stacks of gold ($2520), then full stacks of silver ($1620), then zircon ($780), then copper ($720), and so on.
This will adjust for every new mineral you find the deeper you go.

With this method, the only time you waste per day is
-the time to find surface herbs (which can be removed if you only use mining herbs)
-the time to go to the mine
-the time to go back to the farm