Due to modern technology, a modern economy is far more productive per capita than the Roman economy: on one estimate, the average American in 1998 was at least 73 times more economically productive, in comparable terms (i.e. in
international dollars), than a Roman in the 1st century AD.
[20] Therefore, taxes (and compulsory services) to support the Roman military would have taken a much greater share of surplus per capita production i.e. surplus to the subsistence needs of producers. For the average peasant, the taxes and services he was obliged to provide to the military would have represented a significant share of his disposable surplus.
There is also a great disparity between the costs of the 4th-century army and its 2nd-century counterpart. The much lower remuneration for 4th-century soldiers is reflected in total army costs. Duncan-Jones estimates the total annual cost of the military in c. 150 AD at c. 670 million
sesterces.
[21] That translates into 0.67 million
aurei or 16,800 pounds of gold. This compares with Elton's estimate of c. 47,000 lbs of gold for a 4th-century military establishment of 450,000. Even if the establishment was the 600,000 estimated by A. H. M. Jones, the cost would still be only about a third that for the 2nd-century army.
[22] Such a disparity is difficult to explain. Either the imperial government was collecting far less tax than in the 2nd century (an unlikely possibility, given the numerous complaints about the weight of the tax burden). Or it was spending much more on other sectors. These may still have been "defence-related": e.g. fortifications, irregular
foederati forces, or payments to barbarian chiefs to buy peace and allegiance.