I've barely managed to squeeze just over 1k bc/turn out of my trade routes as the Korx, in ToA huge map. That's including the Neutral Trade research, which btw only gives 1 extra trade route to the Korx, apparently the amount of trade routes you can have also has a hard cap.
Anyhow, this 1k was from all my trade routes combined, each going through at least 4 completely upgraded starbases (including the extra +25% module the Korx get).
This 1k was achieved relatively early in the game, however as time passed by and I actually built a few more trade starbases, I never got a higher amount, it got stuck at +- 1k and refused to go any higher. When I first hit 1k, trade made up about 1/7th of my income (most of the rest was taxes), but when my tax income eventually hit 26k+, trade had still remained the same amount. (I always spent enough money to even let trade unhandicapped by the 10K halves trade mark.)
And this is the Korx, the supposedly awesome traders...
With other races, I'm finding trade even less worth it. Trade was always rather hampered in this game, but now it's downright gimped. I'd very much like it if trade were to become a valuable part of the economy again. Now it is only economically interesting on the very small maps with rare planets, but even then, you're probably better off building warships...