Rich Man's Burden

"Army service was, as stated, a right reserved for citizens, but only to those who were owners of property and other possessions. This was for two reasons: first, the recruits had to purchase their own equipment, and not all citizens could afford that. The second was that a property owner risked the chance of losing his property in the event of a defeat, and so this was considered a good incentive for him to fight more fiercely than a person who had nothing to lose. The citizens with no capite censi (property) were recruited in times of emergency and equipped by the State. In the second century, the minimal property required for approval to serve in the Roman army was worth 400 Greek drachmas."

--- The Roman Wars in Spain: The Military Confrontation with Guerrilla Warfare / Daniel Varga