Just to add a bit of my experience here; I'm playing my first Attila campaign as the Sclavenians. I noticed that as soon as I had a few provinces under my control, my economy is freaking out of control. It's now turn 76 and I've conquered the entire Sassanid empire and all of its puppet stets, with EIGHT doom stacks ready to roll over the Eastern Roman Empire when I feel like it. My income is 44,000 per turn.
Is this how things are supposed to turn out, or did I do particularly well? With the amount of money I have just about every settlement I own is building up towards max every single turn.