So, the fundamental question, I would think, is how much trade is simply necessary for a planet/system, and then how much of that trade is the market providing.
I look to the price modifiers for trade based on the worlds trade codes (from either the LBB1-3 trade system or the Merchant Prince trade system to identify existing trade routes. If a pair of adjacent worlds generate a net positive modifier IN BOTH DIRECTIONS, then somebody will start a 'milk run' between those two worlds. In other words, there will be sufficient demand to support some routine trade.
Looking at the specific trade items in the basic trade (Book 2, IIRC) for which items can be sold with a net negative price modifier, will help identify typical cargo being exported on that trade run.
The same basic concept should work for any trade system.
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