A quick review of this forum did not show anyone with this issue, but I may have missed it. I apologize if this is a known issue.
I had noticed a couple of games where, if I got large cash reserves, my income plummeted. I finally took a closer look at it in my current game. What I found was that, if my cash got over 10000, my trade income would drop by ~50%. No other income or expense seems to be affected. In the specific case that I wrote down, at just under 10000 my trade was 871/turn. When I crept over 10000, it went to 471/turn.
In this particular game I am Dom. of Korx, gigantic map, masochistic difficulty, no custom races, 5 opponents. I've noticed it in other games as other races. Workaround, of course, is to just buy things whenever I get near 10000, works fine, but still...
I am on the current patch - .072 I think it is.
Cheers,
Scott