It's the temples. Brutii gets the one that increases trade income. Couple that with diplomacy (keeping peace & trade rights with other factions) and the Denarii just come pouring in.

I played a Numidian campaign (Mundus Magnus mod) where I built lots of temples to Milqart (icreases trade) and always built bazaars and ports to my cities first. Once I got to a point that I had destroyed Carthage & Iberians and forced peace with Romans, my income just exploded. I had 1,6+ million Denarii when I begun my "war" against the Ptolemaic empire and after I bribed away most of their armies and cities, I still had over 1 million left. Really, my diplomats were an unbeatable weapon