Now that enchanting mats have hit rock bottom I can guarantee you that questing is more money than instance runs. Lately all I do is farm 4 instances and then go to my alts and I really dont have much cash to show for it. Before I was questing on my mains and wow would they come out a lot richer after each day of full questing.

edit: not to mention you run into more gathering stuff, I see maybe one flower and 2 mines per instance.