I saved a bit from building, as I was my own project manager and there was no middle man to make a markup between constructor and final buyer.
But more important: I got the house I wanted built with the materials I wanted and I know what is behind the nice paint and tiles, and underneath the floors. When using a turn-key construction, the building construction company might cut some edges on materials, which you cannot see when you get your keys, but after a few years paints drop off and thing begin to break.
My foreign next door neighbourgh got a luxury house built at same time as mine – the price was double as much per square meter – made as a turn-key project. A month after they moved in the roof began to leak – lots of silicone was used during their first monsoon rain-storm – later they got major water damages. My roof is still tight after 15 years.
After a year the paint began to pieel off, mine still sits nicely on my walls after 15 years. The difference being that I had double layer primer and a little more expensive, and thereby better qaulity paint, coated in three layers onm top of the primer. My neighbourgh got one times primer and two layers of cheap paint (I know what they used, I wouldn't use that on my house).
Furthermore you can chose to use aireated concrete for walls insted of cheap blocks, which gives you both a better indoor climate, but also saves electricity when using aircon. And so you can go on with details.
But, you need to be present when building your house, to use thar method. And you also need to either check up on varoous thin gs yourself – if you don't know about building construction; which I didn't, but and read and asked and listened a lot – or find some advisor.