To address the question to your personal circumstances I suggest breaking it down into three areas – materials, joining method costs and joining method resources.

Materials: copper these days is very expensive, probably 5 times the cost of steel. However if you buy all your steel pipe and plate traceable for composition and process all the way back to the mill, as some insurers require of steel boilers, that will also take extra time and money which would decrease the cost gap steel to copper. You do need more steel by weight than copper, because extra thickness of metal must be used to allow for corrosion with the use of steel.

Joining method costs: Silver solder as used for copper joining on boilers is very expensive compared to steel GMAW (stick) welding rod and electric welding. Gas torches, bottles of gas are also relatively expensive compared to electric welding equipment and operating costs, although TIG electric welding can get expensive if a good quality welding machine is bought, and high quality TIG mix gas bottles are used. TIG does make superior welds on steel if the operator is skilled after much practice. Good results are very possible with relatively inexpensive AC GMAW (stick) electric arc welders, again if the operator is skilled after much practice. On copper, silver soldering gives excellent results if good quality filler rod and flux is used, and sufficient heat is used, again by a skilled operator with much practice.

Joining method resources: For a copper boiler if you were to have one soldered or welded by a boilermaking firm they may use silver solder or TIG process on the copper parts. It will not be cheap, probably in the thousands of pounds cost range. For a steel boiler to be welded by a boilermaking firm it also will not be cheap, but probably less than a copper boiler'costs. Still likely in the high hundreds of pounds. Depending on the welder/soldering firm's preferences, they MAY allow you to cut and prep the parts for soldering or welding, but this must be discussed IN DETAIL up front. Doing your own cut and prep of the materials may save a lot of money if your workmanship is good and you are able to work closely with the welder/soldering firm. If the welding firm do all cut, prep and welding, ie build the boiler complete, I expect costs would be in the thousands of pounds range.

It is also important to discuss your boiler EARLY ON with your chosen insurance firm, they vary in what types and construction of boilers they will insure. With steel boilers they may wish to have several inspections during construction.

It's also best to use a published, well established boiler design preferably with several working examples existing, to make it easier for the insurer to see that the design is viable and proven safe.

Just some food for thought. The above are my opinions/experience only, your mileage may vary. Good luck whichever way you go. JD

That reminds me of a Chuck cartoon in an old ME where he machines a boiler out of the solid!

John