In my opening chapter I have my two merchants arriving at Venice with a cargo of oil and pepper - pretty valuable commodities at the time - but they have covered this cargo with a bit of grain and are telling the customs official that it's "grain... all the way down." As part of the fact checks, I had to work out whether all this is plausible, and whether the actual values of the goods involved justify such deceit.
A ship of this kind (52 tons displacement, 15m long) was found off Bozburun, Turkey. Dr. Matthew Harpster reconstructed the wreck, and he estimated that there were about 1100 amphorae in the cargo hold. Each could hold a couple of gallons of liquid. My estimate is that the same ship could hold about 4000 bushels of grain.
Based on this handy reference site, I calculated some prices. I had to make some assumptions to convert everything: for example, that in this time period a gram of gold is worth 10 grams of silver. (It's more like 50:1 today, I think) Also a byzantine solidus weighs 4.5 grams. And so:
A cask of pepper weighing 20 pounds is worth about 32 solidi: they are smuggling "a few dozen" casks. Estimated worth: over 1000 solidi.
Could not find a reference for oil - but wine was worth 4-8 pennies per gallon. I estimated 2 shillings for the capacity of one amphora... 1000 amphorae * 2 shillings * 18g/shilling * 1g Au/10g Ag --> about 800 solidi.
Claimed cargo: 4000 bushels of grain... @ 50 lb/bushel ... a different source gives a value of 10g silver per 100kg wheat... yada yada ... about 200 solidi.
So, it all checks out: they pay 10% duty on 200 solidi but are bringing in 1800 solidi worth of goods... saving 160 solidi (less whatever bribe they made) 100 solidi at that time would have been a nice yearly income for a minor noble.
So there you have it Dear Reader, I am thinking about all this stuff... it's not all blood and pirates and sex and riots in the street!