Was the objective to establish actual exchange rates, like say 10 pieces of copper for a bear pelt? Were the subjugated people required to deliver and sell a given quantity of these goods? Why trade at all, rather than just demand a quantity of goods in tribute?
There are several possibly interrelated elements within OP's question, so I'm afraid that my following answer will not cover all of them.
First of all, Snyder only discusses the character and significance of the raids by the Vikings, or Rus' (primarily in the 9th century) here (Podcast no. 5) in my understanding, so I'll exclude the discussion about the punitive raids by the Mongols in later period altogether from my answer. You can also check what he says in (34:55-around 37:10): https://youtu.be/36XiKhamtQo?t=2095
In this context, "cities" that they attacked in the middle of the 9th century primarily means nothing but Constantinople, rather than the market towns around the Caspian Sea (that they also sometimes tried to raid in course of the 10th century according to Arabic texts, though I'm not sure whether Snyder also incorporates them in his lecture) or fortifications alongside Russian waterways. It goes without saying that it would be very difficult to get success on repeated tributes taking from such a well-fortified city like Constantionple, and generally speaking, the Rus' (Vikings) in the East failed in occupying the important market/ trading places themselves at least in longer terms.
Then, what means "favorable trading conditions" here?
While the degree of historical authenticity has been disputed (I suppose that majority of scholars, including prominent Russian and Ukrainian ones like Oleksiy Tolochko, now regard the early part of this work primarily as an origin myth of the Rus' elites), Russian Primary Chronicle (or sometimes called Tales of bygone years and compiled in the early 12th century) narrates the alleged Rus' raid in Constantinople in 907 under the leadership of Oleg/ Igor and their initial demand in successive negotiations, in addition to that of tribute:
- "[The Russes proposed the following terms:] 'The Russes who come hither shall receive as much grain as they require. Whosoever come as merchants shall receive supplies for six months, including bread, wine, meat, fish, and fruit. Baths shall be prepared for them in any volume they require. When the Russes return homeward, they shall receive from your Emperor food, anchors, cordage, and sails and whatever else is needed for the journey'."
Put it simply, as Snyder (late in his podcast no.5) as well as Ozawa suggest (Ozawa 2015: 134-37), these stipulations aim to settle the conditions of visiting Viking merchants' status (conditional protection) rather than the trade rate itself. Only if the visiting Rus' had not committed violence both before entering Constantinople and during their stay and stayed as a small band within the assigned quarter with registration, the trade and such a well treatment by the imperial officials would be guaranteed.
The text of Primary Chronicle also inserts two more such Byzantine-Rus treaty allegedly concluded in 912 CE and 945 CE (Cross & Sherbowitz-Swtzor trans. 1953: 65-69, 73-77), with expanded contents like the treatment of ship wrecks and ransom of illegally slaved peoples.
Is it enough to offer supplementary context to the podcast?
Recommended Readings:
Relevant Previous Posts [answered by me, /u/y_sengaku]:
References:
+++