I would really love to see someone address the ancient portion of this question, but I can speak for the medieval portion, at least.
A great majority of "common soldiers" in medieval armies would have had armor, as well. Though focused on a slightly later period than what is usually defined as medieval, I've written quite often about armies and their makeup. The quick version of all of this is that armies in the 15th and 16th centuries were largely made up of volunteer mercenaries, embodied city militias, and volunteer aristocrats, all of whom in varying ways saw themselves as performing a social and religious duty to wage war.
Almost all of them would have come from a social class in which purchasing armor and training themselves in the use of weapons was both possible and encouraged. Owning armor and weapons was, in some cases, legally required of citizens of free cities. There is of course a vast difference between the kind of bespoke, fitted, artistically magnificent garnitures of the vastly wealthy and the kinds of armor city guardsman and mercenaries might wear, but almost all men in medieval armies would have worn some form of armor from at least the 15th century onward.
More of my old posts if you'd like more reading:
How do I get drafted as a medieval peasant?
I am a condottieri, what does my contract look like?
As a medieval ruler, what do I equip my army with?
I am a newly recruited soldier in a 15th/16th century army, what weapon am I issued?
Was fencing meant for sport or warfare in the medieval period?
We might be making some erroneous assumptions here about the make up of the Greek and Roman armor, but if you could clarify the time period you are asking about within Ancient Greek and Roman history, I can give you a clearer answer.
For the Greek hoplite, the panoply minimum was a apsis shield, a dory spear, a psiloi helmet, and a dagger or sword. By the end of the Peloponnesian War that stripped down version was pretty much the standard. During the initial phases of the panoply c.725-650 BC there would have been the bell cuirass, greaves, possibly bracers, and even thigh, shoulder, and ankle guards. It is highly unlikely that that level of equipment would have been affordable for non-aristocrats, but it became increasingly stripped down, until by the end of the Archaic period many hoplites omitted even the greaves. The linothorax alternative to the bronze cuirass would have made the panoply significantly cheaper as well, and by the Battle of Marathon in 490BC, it seems to have been the more popular option.
If you are asking about average panoply cost estimates, the scholarly figures are vague, as are the estimates for wages (c. 1 drachma a day for skilled labor), but I can pull them from my MA thesis for you.
As for Roman armor, it was not as comprehensive in the early republic as you might think of you are looking at the lorica segmentata of the principate. But if you could specify a specific time period, I can be of greater help.