
I've been a fan of David Weber and Sci-Fi for a long time. One thing that I've always wondered about in Sci-fi in general is the concept of interstellar trade. I am not trying to argue to be critical of anyone who says/writes about future interstellar trade. I am trying to understand the concept. Would it be economically feasible/profitable to have interstellar trade?
First of all many sci-fi universes simply take the Terran situation and expand it to a galactic scale. I'm not sure if that logic works. My train of thought is as follows.
Ok, so the only solar system we've studied in any real depth is our own and a sample of one is hardly a thorough study. But in general most Sci-Fi assumes that our system is fairly average. So with a baseline established let's take a look at resources. Our system has a substantial asteroid belt and several planetary rings , the Kuiper Belt and even the Oort Cloud. Our best analysis of the objects in NEO are that many of them are EXTREMELY mineral rich. I have seen numbers thrown around about single asteroids having trillions of dollars in materials and the estimates of the number of asteroids in just the main belt is in the multi-billions. That's a whole lot of stuff to make other stuff. Almost "found lying on the ground" level of availability. Assuming you're not making 1000+ planetoid warships... I'm looking at you Fourth Imperium, in that case you're breaking up whole planets for lunch.
Now we assume efficient/inexpensive interplanetary travel capability (they have interstellar capability after all). That makes resources relatively easy to get and very low cost (again relatively speaking). Now developed systems are assumed to have a well established industrial system for basic and advanced manufactured goods. Only truly specialized goods are not easily made in the average fully developed system and would necessitate trade. But what would the average/below-average system trade to the specialty systems to get these items? I mean the specialty system would be assumed to have a resource pool that includes it's own asteroid belts/ring systems therefore, raw materials would plentiful. Manufactured goods? Well, unless shipping costs are practically zero it would be cheaper to make it themselves. But we know shipping isn't almost free, freighters can take weeks/months to get from system to system in most sci-fi universes, have reasonable sized crews, huge capital investments in the vessel, and ongoing maintenance of the vessel. "Labor costs are cheaper in less developed systems" you say? Even today, automation is making manufacturing less and less labor intensive, plus just about every sci-fi universe has some variant of AI/robots/nanobots that would facilitate inexpensive local manufacturing.
The only thing I can think of that a new/underdeveloped system could offer a fully developed system are biological resources. Wood or wood analogues, vegetables, fruit, meat, spirits etc. All of these things would be unique to the system of origin due to the influence of the unique biosphere on the product. Even the type of star would have an affect. A steak from a terran cow raised on a different world would have a different flavor/texture that might be appealing and worth transporting. However these items would most likely be luxury class goods once transit costs are added in and not likely to be a big enough "cash crop" to allow for development of infrastructure and economic growth.
So that's what I don't get. Given my above train of thought, why would there be any substantial interstellar trade?

Comments and discussion on why I'm wrong and why there will be trade welcome! After all, without trade there are no pirates.
