As best we can tell from Pseudo-Aristotle and Herodotos, the fundament of the satrapial economy was a tax on arable land, which was then supposedly complemented with taxes on... pretty much all commercial activity (how effectively these were levied is another matter). Tribute was also paid in the form of manpower, labourers, goods, and so forth. The success of the system is evident in the massive construction projects undertaken in the reign of Dareios, who is generally credited with these reforms.

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It's a bit hard to suss out of the sources where the various revenue streams went, but it seems like the Great King primarily took in revenue (& tribute) based on land, whereas commercial activity provided revenue on a city- and provincial level (at least, this is the only plausible way I can parse the source material, - it's hard to imagine the big guy in Persepolis keeping tabs on the ups and downs of all commercial activity in Ionia). Moreover there appears to have been a "hospitality" system present - when the Great King visited a province, the personal wealth of the Satrap and the resources of the province was put at his disposal, with the expectation that this would be repaid in royal favour (e.g. marriage to one of the king's daughters). The administrative documents of Persepolis add somewhat to the confusion; preposterous amounts of wine and grain are doled out to high officials, with no explanation of what they do with it or whether it is actually doled out in natura or whether they simply measured value in terms of such basic products.

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I don't think one can really speak of an "Iqta system" or a "feudal system" except in some idealized form somewhere, and my knowledge of either is not great. But if there was something peculiar about the Achaemenid system, it was probably the enormous weight vested in the Great King's person, and the uniqueness of the Achaemenid line. The Great King really was the most powerful man in the world, quite unlike the king of High Medieval France, or the Holy Roman Emperor's theoretical position as the King of Italy.

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I think Pierre Briant's chapter on the economy in _From Cyrus to Alexander_ is pretty good, for further reading.