I don't know how it's handled in Wizards' D&D, but AD&D always included a history lesson and differentiated between Field Plate, Full Plate, and Plate Mail. In that sense, it would depend on the tech-level of your campaign. The affordable steel plate armour you're describing, like munition armour (half-plate in D&D) or cuirass breast plates, didn't arrive on German battlefields until the Thirty Years' War (1618-1648), which is the latest era of setting recommended in 2e rules. Earlier depictions often show cheaper varieties from leather. Wikipedia even states that "During the English Civil War (1642–1651), only the wealthiest and physically strongest men could afford this type of armour. [
Cuirass]" Thus, if the standard setting is dark ages, crusades, Hundred Years' War or Renaissance, metal plate armour was unaffordable for common soldiers. With the advent of firearms during the Renaissance, plate mail disappeared and then had its own renaissance with the evolution from unwieldy heavy plates of wrought iron to standardised lighter steel versions at the beginning of early modern times. If you're going for that Dark Souls suit that allows you to roll around, climb walls or mount a horse by yourself, you'll have to pay an individually fitted field plate or full plate.