Re: Above is just for ASME cert

I've heard some of your pedigree, so if my answer sounds simplistic, excuse me, the comments are to help elucidate those who have less experience.
The first part of the answer about high cost is "Because they can" Most model builders are private entities of traditionally somewhat modest means who would just walk away if the cost was too high. Many restorers of full size equipment have a little deeper pockets.
The second part is the extra equipment and such needed to handle the LARGE pieces of steel in a full sized project, as well as the space needed to house it.
A third part is liability, most models are over engineered, and used only intermittantly in sparcely crowded areas. Full size uses a smaller factor of safety to save material, and (in theory) is to be used more heavily, and around the public. Models also generally run at a lower pressure. So the potential for large losses in event of failure is greater.
A fourth part is that there are certain parts of the code that models are exempted from (by special rules) that simplifies construction somewhat.
All those add to the costs. You CAN possibly save, if you can do much of your own work. There are computer programs that will do much of the calculations, and newer non-destructive testing methods like x-ray and ultrasound testing can reduce some of the liability worries.
Much of the headache in opening a new ASME shop comes with the redundant engineering costs involved because there is no such thing as an "pre-approved" process, joint, or design. Each shop has to submit every basic step for approval...even if they are doing exactly the same thing in the same way. (At least they only have to do it once, unless they change the basic perameters too much). Then you need to pay for all the welder testing, inspections, insurance, and fees BEFORE the ASME decides if you get a stamp.
A big problem with the ASME guidelines (I think), is you have a code that was, and still is, rather a hodge-podge of sometimes conflicting, somewhat arbitrary rulings, patchwork stopgaps, and quasi-political kneejerks, granted, many were stuck in there AFTER somebody discovered that something just didn't quite work (can you say "BOOOOOM!"?). All that is overlaying a framework of century outdated metallurgy....picture a closet where you keep cramming things in, but rarely take anything out.
I do seem to remember the wonderful promises of a new, and straightforward "unified steel code" that was due out by 2000. Last I heard (albiet a couple years ago) it was mired down, not so much over technical problems, rather mostly turf fights and brush fires by people who were afraid of the erodsion their power base, hiding behind "public safety" issues. In the end it may, if/when it actually is put in place, be "unified", but straightforward may just end up falling by the wayside.