So I figured out a more simpler way to make money while idling while doing it by yourself.
Best to do this before destroying the blocks achievement.
I used the same custom map for destroying blocks but I set it up with steel blocks. Every time you mine a steel block you get $10. The miner doubles this.
1. Create a map with respawnable blocks
Create respawnable steel blocks, I used the following setup: 5x5x5 steel blocks
2. Set up an online lobby (Custom map which you just created) with the following settings:
DEATHMATCH
TAB 1:
Max Players: 2
Private Slots: 1
Timed Match: OFF
Match Length: Doesn't matter
Score to win: Doesn't matter
Infinite resources: OFF
Destroy blocks: ON
TAB 2:
Lives: 1
Block strength modifier: 0
3. Choose first stock class: ASSAULT (IMPORTANT, this class has the MINER bonus)
4. Go sit against the cube of blocks and aim towards the blocks, make sure to go in BUILD MODE
5. Activate turbo controller, keep spamming the button
6. When you want to end the game you can suicide to save the game and progression
I made about half a million dollars in a span of four hours or so.