82% - the final tier of civic research gives you -10% (and in a few turns, I'd get another -3% from the final civic tech).

In places like Liguria, Venetia and Magna Graecia, with the governor bonus, a spy investigating corruption and the ecumenical matters edict, it goes down to 53% (which is why Magna Graecia and Venetia, when fully upgraded and optimized, can yield about 10K to 11k by themselves).

Corruption really has never been an issue. I suppose it's harsher when you're playing a barbarian faction and your expansion brings corruption levels from nothing to around 80%, stalling your income as you expand. For the WRE, it's stable. You just don't expect buildings to add their full value, but concentrating on a province is still very worthwhile.

I've had about 15k to 20k income during the majority of the game, probably peaking to around 40k just before my big push against Persia. The trick, for the Romans anyways, is that food is paramount, industry generally sucks. Best provinces are those with extra food sources: ports, olives and wine (dried fruits). Focus on those, and you become rich.