The best I ever made was back in Wrath running heroics and looting everything. Then using the badges to buy epic gems and orbs to craft items. I had one toon skin or mine mobs. Anything under a 7g value got disenchanted, the rest got sold to a vendor. At it's peak and the right heroic (nexus, occulus) I could pull in over 4k.

My recommendation would be to run instances and loot everything. People still buy low end leather, cloth, enchant mats. You should have a nice pile of gold when you hit 75 and you spent your time leveling as well as building a bank roll.

One thing to keep in mind. You will never make as much money with level 75 toons as you will level 85 toons. Access to dailies, heroics, and the ability to farm items that people want will be out of your reach. But I am sure you are going to have a 5 or 10 man team get to 85. Shoot a level 75 toon doesn't stand a chance against a normal mob in some zones.