I was wondering if there's a given gold drop rate formula based on the rarity/hp/level of monsters and if anyone might happen to know what it is.
I can't say I know a particular formula, and I'm sure that an official source would never divulge it, but from what I am noticing it seems that the "gold ceiling" continues to get higher as you progress in difficulty.
For example, in Act I - Normal difficulty you may never see a gold drop higher than 60 gold or so, and they can be as low as 1 gold.
In Act I - Nightmare difficulty, I have seen gold drops as high as 300 gold in just the first half of the act, but have also still seen gold drops in the single digits.
My theory, is that the range expands to include higher potential values, but it is random anywhere within that range, meaning you can still get 1 gold drops later on but you could just as easily get 1000 gold drops.