Well, as a bit of reality checking, it's definitely not the biggest economy cheating in TBS, Civ2 has that title down with no competitors. Cities there didn't even build there stuff (or maintain it), it was materialized using dice rolls. If you've ever had a size 2 city with all its squares blockaded in Civ2 only to have it produce one starship component every 2 turns you know what I mean. And this cheat was global throughout all difficulties.
At levels above Intelligent the computer not only gets flat bonuses, if I understand it right they also get bonus attribute picks for their race. And of course they also have all the standard boosts that come with the game. So you get a comp AI with a racial +economy, a bonus to economy based on the difficulty level, a resource mining bonus, and a fully developed planet infrastructure and it's not hard for me to believe it could reach some really amazing numbers.
A better approach for this kind of thing in the future would be simply to report the numbers and ask Brad for an explanation, possibly they -are- too high (there are bugs in the game after all), but they might be dead on as well. I just pulled up a saved game I had, this is near the end of the game with all tech research, my biggest planet at size 39 is doing 163 BC income so the value you reported for an AI with additional picks and bonuses seems about right.
If you want the most even game possible, play on Intelligent, the higher difficulty levels are for those who are too good to be challenged by the AI in a fair fight, so the AIs get some significant boosts not in the spirit of cheating but in the spirit of providing better game play for those who are too good to get much enjoyment out of the lower difficulty levels. Yes, it's ridiculous, it's supposed to be.