I've set Gringott's decimal digits setting to 0 and MobHunting's reward_rounding to 1.
(And my denominations are: 1 = gold ingot, 9 = gold nugget, 81 = gold block)

I then decreased the amount of money the mobs would drop, like this zombie setting:

  zombie:
    enabled: true
    message: You killed a §1{killed}
    money:
      amount: 1:3
      chance: 0.6

Then I killed a zombie, and I got this:

Zombie dropped 5 Gold Nuggets, 3 Gold Ingots!

I thought I'd get somewhere between 1 and 3 ingots, 60% of the time I kill a zombie.
Any idea what the issue could be?

My bad! Turns out ranged amounts have to be entered as strings, so '1:3' instead of just 1:3.
MobHunting would actually default back to the original values and write them back to the config file, too. That default value was around 64.

I fixed the syntax and now everything's working fine!