I've been doing some statistics back in the beta about gold drops and now that I've seen the full game I've formulated a theory on how gold balance works.
It's pretty simple actually, quite a bit different from how diablo 1 and 2 did it, but makes a lot of sense given the auction house and real money aspects of the game.
I think gold drops aren't balanced monster by monster, but on a much higher level, possibly even act by act. What I've seen in the beta is that gold drop ratios are perfectly linear. This means no matter how fast you go or how many monsters you kill, you only get a fixed amount of gold in a given time.
If you've ever blasted 20+ monsters with a single shot and wondered why they only dropped 3 small piles of gold - this is why.
This is actually interesting from a class balance aspect as well - it makes all the classes equal when it comes to gold farming. The only two ways to increase your gold income (from gold drops) is by using +gold % items or going into a higher end area of the game.