In un-modded MTW/VI, I've gone over 10,000 in numerous provinces (Const., Antioch, Egypt, Sweden, etc.), although it's certainly not easy.

The highest I can recall was when I was playing as the Egyptians, where Antioch was giving me almost 14,000 florins/turn just by itself. On top of that, Egypt and Tripoli were both pulling in another 20-21k as well, meaning that all 3 provinces contributed around 35k to my treasury every year. (Man I love playing as the Egyptians!) If you want a large trade income, just pick any faction that sits on the eastern and/or northern shores of the Mediterranean.