You know... if someone wanted to do an semi-in-depth analysis of this and put together details on such pricing and costs and wanted to write up an article on the subject for the
http://www.guildcompanion.com/, I suspect it would be a well-read and useful article... hint hint
Regardless of that though I think the key to such info is to try to be sure that the prices/costs are not too high either. One thing to keep in mind is that different GM's may want to include such higher tariffs, gate-use tolls, docking fees, and bribes; while others may assume that such things are relatively cheap and low and not bother with them. So to some degree those costs and prices may need to be varied somewhat based on the GM and their interpretation of the setting.
Another thing to keep in mind is that there will likely not only be some variation on tariffs and such based on what the shipment is, but also on what is charged for such shipments (things that have higher tariffs will tend to have a higher shipping charge, things that require more handling (in transit or in loading/unloading) may have higher charges involved as well. As an example of this (someone had made a firefly reference earlier, or maybe that was in a different thread on a similar subject), when Serenity was used to ship the cattle, that added a bit of additional work (not only in handling, but also the cleanup after they were unloaded, as I'm sure Jain wasn't happy about having to hose down and squeegee the cargo hold deck afterwards).
Likewise not all freight is of similar density and thus doing an across the board mass measurement may not always be the best way to handle it (for example, some of my employers many products an entire semi-load weighs only a couple of tons, whereas some others you can't fill up a semi with the product as it would exceed the weight limits on the roads (many of these are often times shipped by rail when possible). Likewise some of our products need special handling due to the extremely hazardous nature, which drastically increases our shipping costs.
So there should probably be some built-in variability to such numbers for some types of freight for various reasons. But it would be good to have such values defined, even if only loosely.
But this is an interesting discussion so far on this subject.