I'm unfortunately woefully undereducated on the state of the Sassanid economy. I know there's a shortage of records, but surely there must be some archaeology on the subject?
Did the Sassanids tax? How urbanized were they? How much of their economy was based around trade or agriculture? Etc.
Sorry, the China debate has drained away my willpower for tonight, but I can very warmly recommand Peter Christensen's The Decline of Iranshahr (a good part of which is available freely on Google Books); it includes a good (economic) survey of the heartland of the Sassanid Empire, Mesopotamia. In addition to this, Howard-Johnston's great article “Two Great Powers in Late Antiquity: a Comparison” can provide you with a good overview of the Sassanids' internal organisation. And to answer shortly your question: they had apparently a pretty functional taxation system, and a very organised irrigation system in the East. The so-called “Red Snake”, Iran's Great Wall, is a pretty good proof of this power.