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Using a long series of estimations whose details can be found in Rick's article he concludes that the annual operating cost for an airliner is about million (not counting fuel, landing fees, and taxes).
An airliner's purchase price is 0 million so one year's uses costs about one-third of the purchase price.
Sometimes these trader cultures in large clan-ships have a Thalassocracy, where they have a monopoly on trade since they control all access to space.
You can alter some of the assumptions yourself to tweak the economy to suit your science fictional background.
Merchant Starship Costs Assumption: starships in the interstellar empire are equivalent to present-day jet airliners.
Don't forget the entry in this website about Cargo Holds As mentioned below, if you want to play around with interstellar trading, or even try doing a full simulation (to do worldbuilding for creating the background of your new novel), I'd suggest getting a copy of GURPS Traveller: Far Trader.
Written with help from a real live economist, this allows one to model interplanetary and interstellar trade with equations and everything.
Assumption: starship purchase price will only be about $1 million per ton of cargo capacity instead of $2 million, because starships are orbit-to-orbit, need no landing gear, need no wings, can use lighter structure because they accelerate under 1 g, and we will assume they can carry twice as much cargo per deadweight (inert mass) as a cargo jet.
Assumption: cargo starship operating cost is similar to cargo jet.
You'd probably be better off reading the full article but some people want executive summaries.
Rick starts with certain assumptions and follows them to various conclusions about the interstellar economy.
Therefore it costs 0,000 per ton of cargo capacity per year to run a cargo starship.