Trade centers alone don't produce income, you need 2 to get anything. the trade ships make money when they stop at a trade center. so every planet with at least 1 trade center in an entire star is one hellish economy and you can easily place bounties in the millions on your foes. not to mention those arid colonies. for the most part i only have shipyards on the front line planets the rest have trade centers and refineries. 2x orbital refineries on an ice planet = lots of crystal. if your playing a full galaxy set up(100 planets, 4 stars) with one star under your belt, nothing can touch your economy. save for pirates.