Maybe he means the Lorica Segmentata. The more stereotype picture one has whilst mentioning a Roman soldier.
The Segmentata wasn't used by the time and it was used just a small period in the roman history, cause it was to expensive to made.
You would never see the Segmentata in a roman civil war..
The Hamata and the Montefortino helmet was the best outfit for a soldier
Earliest evidence for Lorica Segmentata were excavated by archeologogist in Germany, at the augustean battlefield of Teutoburg Forrest (9 AD). Before that we simply have no scientific, reliable evidence for this kind of armor (I'm archeologist, so I know shit about that).
But no, the lorica segmentata was not more expensive to make, in fact it was easier to make. But it was very difficult to maintain and must have been tailored for a single person. It couldn't be reused by another person, because this armor type needed to fit perfectly on the body to be effective.
Reasons of disappearance were, that the maintaining was too costy and the fighting technique changed in the 3rd century. Segmentata was designed to withstand the piercing of a Gladius, but since the mid of the 3rd century, even the infantry started using the slashing longsword Spatha.
And to be precisely, Lorica Hamata never got displaced by the Segmentata at every period the Segmentata was used. Hamata was still in use by cavalry, auxilia troops and even legionaries itself.
But Hagenau/Coolus type and Waisenau/Imperial type Helmets were started using in the ceasarean/late civil war period. Especially the Imperial type, made of iron instead of brass, was a celtic invention adopted by the romans in this period. But Montefortino type stayed the main helmet until 0 AD.