When your treasury is 20,000 or greater, you receive an income penalty of 20%. Although I don't remember if anyone has ever verified this, it is supposed to factor in the same way any bonuses would-so for instance if you had, say, +50% economics, it would lower it to +30% (rather than 150% x 0.8 for 120% or +20%).

In other words, the higher your economic bonus (from racial points, techs, and mining starbases), the less it will matter, though it will still be less than if you were below the 20,000 mark.

Thanks,

But where this info is in documentation?

Ah, that's good to know. I couldn't figure out why on my last game my income kept jumping around from +500 to -200 every other turn. It was really strange to be in the yellow, rush buy a building, and see yourself green again. Until now, I assumed it was the taxes being lost from filling transports