Part of the problem is that prior editions were in fact discrete lots of 3 size ranges:
Incidental: 1d6 Td
Minor: 1d6 × 5 Td
Major: 1d6 × 10 Td
Freight in T5.09 is one roll per day for freight, one for mail. Cargo is a flat 100Td - but should it be?
In order to answer that question, I'd have to go back to Merchant Prince or, perhaps, T4... tracing back through the rules to see if this came from left field or not.
When it comes to trade, I prefer rolling directly for things, and letting those things modify the price based on what they are and what world they're from. Surely there is an essential set of trade goods that work well in any setting (and it seems that Book 3 was a good attempt). Now THERE's a thread.