Re: Above is just for ASME cert

Not really; FRA specifies it must meet a code when it's built and that any repairs or modifications to it must also meet a code. FRA rules are not a construction code or a repair code, only an inspection code. The only applicable construction code is ASME, and the most applicable repair code (gow to fix it correctlyh)is NBIC.
Yes, there is labor in the design, though if you are building a boiler as a replacement for an existing boiler, it's more a matter of reverse engineering as far as most dimensions are concerned. There there is doing all the calculations to be certain it will meet code, then converting all this into CAD for the fab shop that will be doing the cutting, punching, bending and welding. This can be time-consuming, and time is money.
The code requires certain material specs be used. All of which are readily available items in the steel industry - for example, SA-285 or SA 516. The material must be tested and certified and a paper trail developed and maintained on it.
Fab labor is a fraction of what it used to be, what with modern shops with modern material handling systems and CNC machines that use only one or two people to do what whole gangs of boilermakers and laborers used to do, and do it much faster, as well.
Given the materials and the engineering data on a disk in CAD, a good fab shop can cut, punch, drill, trim, roll, fit and weld a complete boiler and firebox in a few days, not counting driving the staybolts.
The bigger and more complicated it is, the more it will cost. But when you spread the cost ovet the expected lifesapn, it's pretty cheap.