Free WoW Guide: Easy gold at level 40

In the Scarlet Monastery, at the Graveyard:
You'll need a friend.
The first room, with the torturer spawns a chest that usually has at least 3 gold (or 3 gold worth of items in it).
Get the part where you kill the monsters down pat, it should be fairly easy. Even higher levels should be able to literally walk in and do whatever they want.

The fun part comes when you need to reset the instance to do this again. That's where your friend comes in:
* Group with your friend, enter the instance, and kill things, get the chests, and then, before leaving the instance, disband from the group.
* Invite the same person back into your group.
* Leave, the instance has reset.