Re: Above is just for ASME cert

I think the key phrase is "... boilers should be relatively low cost."
IMO, they are relatively low cost. Of course, we could discuss relative to what, but when one considers the impact of inflation, then compares the costs for a new boiler 50-60 years ago (example: ATSF bought new boilers from Baldwin and A.O. Smith for some 4-8-4's and 2-10-4's tht had been built with nickle steel boilers and caused endless headaches), I think one would find that the costs today are about the same or even less.
One thing that drives the engineering and manufacturing costs up is that locomotive-type boilers are an oddity in today's world. They are nothing like the stuff being built daily for all other applications, thus it takes man-hours to convert, say, Baldwin's drawings for a cab-forward boiler and firebox into machine language that CNC machines can understand. And it takes machine time to make the odd parts, such as syphons and circulators and throat sheets, all of which are far more complicated than they look.
Steel is cheap, even the proper spec steel. Sometimes you might have to special order the exact sizes and thicknesses you need, but steel companies (and fab shops) are hungry right now and are more willing to roll and sell you the odd size stuff than they would have been a year or two ago.
As for the ASME code, yeah, it's a pain and so is the organization, but for practical purposes, they are the only game in town. Just try getting one certified (or getting it insured) that isn't built to ASME code and see what happens.
A new boiler and firebox for, say, a good-sized 4-8-2 or 2-8-4 might cost as much as $800,000 to engineer and build and install; maybe less, depending on how complicated it is (Belpaire fireboxes, syphons, etc. cost extra). I still think that when the costs are spread over 100 years or so of expected service life, that isn't bad at all.
Then think about the fact that, if you have this new boiler and you operate and maintain it properly (don't thermally abuse it, wash it out regularly, keep your water treatment up to snuff, etc.) it will give you VERY low-cost service (compared to one that's already 75 years old) for the first 15-20 years; since everything is new, you probably won't even have to so much as replace a broken staybolt for the first 10-15 years of it's service life.
Also, new diesels cost around $2.2 million apiece, and that helps put it into perspective.