Also taking into account the needs of game rule simplicity. I don't want the GM to do lots of calculations whenever a Beowulf gets to port. There are enough as it is.
In a trade-significant economy, I think the MegaCorps have everything tied up with long term contracts and relationships. We little guys are not going to compete with Fedex, or Naasirka.
Besides, if I were Naasirka transporting goods for my luxury goods subsidiary, I would charge cost plus 10% and tie it up with a long term contract.
In a frontier economy, I think the Traders' Guild and local worlds get together to establish standard rates. To encourage trade, facilitate planning, and to prevent (or discourage) gouging. That is currently Cr1,000 per ton per jump. Enough to make money, but requires work. Requires speculation on cargos to make ends meet. Luck, hard-working guys ultimately get rich. Lazy, unlucky guys don't.
Rich guys have good ships and ultimately negotiated those cost + 10% contracts and graduate out of free trading.
So the real key is our term Free Trader. A ship/merchant who carries goods in "international" trade. Like the sutler's wagon following an army, or a merchant's wagon selling to the indians or the settlers.
That implies that the money to be made by a Free Trader is in speculation rather than freight hauling.