Druid gold farming is very spec dependent.
Feral is by far the best choice for druid gold farming. Its extremely good for grinding.
Feral grinding is similar to rogues. You start off in stealth and stun the mob with pounce. Then follow up with feral fairy fire and mangle for the debuff. And if you had full energy you can squeeze in a shred before the mob becomes unstunned and turns around. This usually gets them to around half HP and is easily finished with a few mangles.
In my opinion getting Improved Leader of the pack really cuts down on downtime thanks to that extra heal on every critt. It pretty much lets you farm forever without having to take any brakes.
Also there is no quivers or consumables taking up bag space. So its pretty much the perfect farming class.
Now the Moonkin spec has the same mana issues like the mage and Shadowpriest class. But its still pretty quick and easy and if you have a feral weapon you can use it in moonkin form to melee back some mana. Although i think thats a bit inefficient. My trick is to go do 1 Alterac valley battle ground before i go farming and get a stack of 80 free awesome mage waters. That solves any costs associated with mana.
Resto spec is pretty much useless. Best bet is to put on a set of feral gear or Moonkin gear and try to make the best of it but that’s usually only good for quick dailies that require some mob clearing. I would not recommend farming for longer periods of time since its very inefficient.