Obviously it saves on design cost; but what ratio of total cost is design? I would guess steel, site prep, and transportation are the largest chunks of the pie.
You're right, but I would wager on something like a coaster they're not
that much less than half of the pie.
In my field (building design), it’s often stated that for a relatively standard building (i.e. not a huge bespoke airport in the Middle East, but a mid-size office/residential building) the design cost is around 30% of the total building cost.
In this instance, the design cost would cover everything – geotechnic, structure, civil, services, fire, acoustic, etc. Basically, everything you’d need to generate a set of plans for a contractor to build off (before anyone says, I know I’m simplifying things a little as there are contractor designed portions and all that kind of thing). This typically is said to be around 30% of the total building cost. This work would (with the exception of some geotechnics and very small input from the rest) not need to be repeated if you decided to make an exact replica of the building.
From there, everything else you have to pay for every time you build a clone. Groundworks, materials, fabrication, installation, commissioning, all have to be done every time. This is typically said to be around 70% of the total building cost.
The thing to remember with buildings of course, is that they use a LOT more materials than coasters. Simply by virtue of their form. The material mass of a building must be orders of magnitude more than in coasters, and a lot of that 70% is accounted for in the purchasing and utilisation of that material.
I would be willing to wager coaster design is at least that sort of ratio of design/construction cost, but wouldn’t be surprised if the split is even higher, maybe up towards 50%. Not necessarily because I think the design time is that much longer (don’t underestimate how complex buildings can be), but because the material cost, fabrication time and construction time are that much more reduced.
I’m trying to use my experience of one part of the engineering world to inform judgement on another, which is always risky, but my “gut feeling” (for whatever that is worth), is that building a clone saves you a lot of money - in the order of 30-50%.