Supply contracts with the military generated trade with producers near the base, throughout the province, and across provincial borders.
The Roman navy contained probably 30,000–40,000 marines, sailors, and oarsmen, of which 15,000–20,000 in the Mediterranean fleets at Misenum and Ravenna (contrary to popular belief, Roman warships of this period were not rowed by the forced labor of convicts or slaves, but by volunteer professional oarsmen)[16] and perhaps the same again in the classis Britannica (English Channel fleet) and the fluvial flotillas on the Rhine and Danube.
At its peak under emperor Septimius Severus (r. 193–211 AD) therefore, the standing Roman military establishment may have comprised over half a million effectives.
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 if situated within reasonable distance of the units.
Compulsory services performed by the richer members of society helped to supply military needs (and maintain the infrastructures of cities and the empire, in effect a tax that was paid for in money or labor).
These additional costs may still have been "defense-related": e.g. fortifications, granaries, armories, clothing factories, irregular foederati forces, or payments to tribal chiefs to buy peace and allegiance.
Elton points out that his estimates are for very base pay, 9 solidi for cavalrymen and 5 for foot soldiers, allowing 10% additional expenses for officers' salaries and do not include non-quantifiable military costs such as fortifications, ships, artillery, wagons, tackle, armor, uniforms, pension payments, allowances for families, periodic donatives.
By comparison tax officials calculated the annual cost of a soldier at 36 solidi covering pay, provisions, and equipment.
Even if inflated to boost gold income to pay mercenaries as replacements for Roman recruits it suggests a wide range of hidden costs.
The discrepancy may be the result of lower pay scales, greater quantity for equipment needs as seen in the establishment of over 100 imperial armories and clothing mills, cost-cutting measures, the expenses associated with frequent troop movements and costs other than payrolls, animals and other easily quantifiable expenses.