>If they'd had a cheaper construction method that they estimated would last only a hundred years, say, would they have used it?
I believe they did have one and did use it - timber buildings. I think most ancient societies used timber but we don't see it now because it doesn't last. There's an article here on the find of "layer upon layer of Roman timber buildings, fences and yards" under London.
I know about their timber constructions, but I'm pretty sure that some of the things they built that have lasted couldn't have been built with timber, like the aqueducts, the Pantheon or the Colosseum. It's those things I am wondering about.
I believe they did have one and did use it - timber buildings. I think most ancient societies used timber but we don't see it now because it doesn't last. There's an article here on the find of "layer upon layer of Roman timber buildings, fences and yards" under London.
http://www.bbc.co.uk/news/uk-england-london-22084384