Easy Farm
Step 1:
Get the Cursed Skull. This will triple all gold value
Step 2:
Go to Carnival 1 (Level 1-1), and play up to this part, collecting all the coins in the level. then save and quit.
Alternative Step 2:
You can go to Swamp 4 (Level 3-4), and pick up the coins at the start of the level, and then save and quit.
With this, 999 coins is doable withing a few minutes!
Enjoy!
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2858539836