Roman architectural revolution
en.wikipedia.org
en.wikipedia.org
One of the reasons why you see "drill holes" in the Coliseum is that was how people were extracting the valuable metals.
Of course, once the clamps and rebar were extracted, the Coliseum was now quite vulnerable to earthquakes.
It was used to fix the clamps/braces to keep stones together in the stone masonry, the clamps themselves were steel alright.
Rebar in modern concrete is mostly to provide stength in tension as opposed to "compression" (which concrete is quite strong in). Modern building then relies on this strength.
The lead rebar in the Colisseum was mostly to keep blocks that were laid in compression the ability to stay locked and aligned even if they got shifted a bit.
Typically used diameters in buildings (Italy, Europe) are from 10 to 24 mm, but 6 and 8 smaller diameters (mostly used in electrowelded nets or in smaller pre-cast manufacts) and larger diameters 26 28 30 32 mm (only used in large structures like bridges) do exist.
https://www.oppo.it/tabelle/tondi_sezionexnumero.htm
So, a range from 0.2 to 6.3. kg per meter.
Cost is (aproximation) between 1.5 and 2 €/kg, it depends on a number of factors and is a fluctuating one, though it is usually the same in a same country, without much difference on the specific area.
Typically a foundation will have between 60 and 90 kg of steel per cubic meter, pillars and walls between 80 and 120, and slabs/beams between 100 and 150.
Cost of concrete is even more varied and it is heavily (even in the same country) affected by the cost of the local aggregates (sand, gravel, etc.) so it is hard to make a ratio valid everywhere, but surely it is much higher than what you hypothized.
If we assume that price of concrete (not reinforced) is 100-120 €/m3, this ratio to rebar cost is at least 1:1, but could go up to as much as 1:3.
https://news.ycombinator.com/item?id=31834833
https://news.ycombinator.com/item?id=29377799
Currently the trend in cement is to add pozzolanic components to portland based cement in larger quantities to combine the best of the two (fast setting and high resistance of Portland with resistance to water and cracks of Pozzolanic).
Actually, most layman do care about it, but short-term KPI-driven management (in the private and public sector alike) don't, because as you say they don't have the proper incentives.
By the way, laymen did and do pay for buildings made out of brick which can last for 100s of years.
Part of the problem is information, in that it’s hard to know how long things will last. But I think this is secondary to most people being short term focused. I’d love to see a building company offering special materials for an extra long design life, but I doubt many people would be interested.