I've been doing some reading on this subject, as it interest me. I'm posting my opinions and thoughts here, if I'm way off base, I'm sure some kind soul will come along and set me right
For something like the bed of a lathe, cast iron is about the best cost performance material you can find.
It is easily produced, and machined, it has a high strength to weight ratio, and once a design is made, this casting can be repeated many many times, so your investment pays off, that being said, changing or tweaking the casting is not a big deal, as the form that the mold is made from can easily be altered. For example, lets say a certain part of the casting, over years of use, starts to fail, a section or an area that was just a bit too thin, well, this can easily be updated on the original form, material added to make the casting a bit stronger etc.
Basically a good cast iron bed is a lot easier and cheaper to make than a good steel bed for a lathe.
The steel beds on a lathe like a Oneway or Robust take a lot of VERY precise steps in the assembly and the welding of the lathe bed, then some machining. On the welded steel bed, every weld has to be just about perfect and the weldors who do this kind of work cannot be trained up in a few weeks, they have to be VERY good weldors indeed. If a mistake is made, the whole deal is trash, start over.
With a cast iron lathe bed, the majority of the noodling work goes into the design of the form that is used to make the mold, if that is all done correctly, then you are just making copies of that form. Once the cast iron piece is out of the mold and seasoned, it gets machined, again, not a real high tech operation there either, as the majority of the work will be removing the casting marks and then machining only a couple of surfaces.
If a piece of cast iron is not poured correctly, has a cold pour, or such, you just smash the piece up and add it to the next post of iron you are melting up, recycle!
Cast iron is also good as a lathe bed, as it has a fair bit of elasticity, in the lower areas on the strength scale, (once you get the tensile strength of cast iron up higher, the elasticity falls).
If you build a well designed lathe bed, with plenty of mass in the right places out of cast iron, you will have a cheap solid lathe bed.
I'd say this must be true, as so many of the machines on the market for the last 100 years have been made from cast iron. The steel bed lathes are still "Johnny come latelys", nothing wrong with that, as for a well designed well built steel lathe is very hard to beat, for rigidity and other aspects of the design that I'm sure I do not grasp, but they do cost a lot more, and are more difficult to produce, needing a much higher level of skill in the craftsman doing the work.
Yes, and interesting subject.