Why sheet metal is the cheapest way to make a large part until it is not
Where the cost curve turns, and what turns it.
A large part cut and bent from sheet metal starts out as the cheapest way to build it precisely because the process needs almost no dedicated tooling to get going, but that same advantage flips into a disadvantage once the quantity climbs high enough, because every single part still has to be cut and bent individually, one at a time, while a tooled process paid for once starts stamping out finished parts with barely any labour added per piece.
Why sheet metal has almost no starting cost
Cutting a part from flat sheet with a laser or a waterjet and folding it on a press brake needs nothing built specifically for that one part beyond a program file and a set of standard, reusable tooling already sitting in the shop, the same brake dies and the same cutting head serving one part today and a completely different part tomorrow with no retooling cost charged to either job. This is why sheet metal is so often the first choice for a new design, a low-volume production run, or a large part that would need an expensive, custom-built mould or die to produce any other way, since the entire upfront cost most other processes would demand simply does not exist here, and a designer can go from a finished drawing to a finished part without anyone having to commit money to a tool that only that one part will ever use. This flexibility carries a real, ongoing advantage beyond the first part too, since a design revision after the first batch has already shipped costs sheet metal fabrication little more than an edited program file, where the same revision arriving after a dedicated die or mould has already been built and paid for can mean scrapping and rebuilding a genuinely expensive piece of tooling from scratch.
The rented-drill comparison
Renting a specific power tool for a single weekend project costs a modest daily fee, considerably cheaper than buying the same tool outright for a job that will only ever need it once, and a sensible person facing a one-off job rents rather than buys for exactly that reason. The calculation changes entirely if the identical tool turns out to be needed every single weekend for the following year, since the rental fee, paid out again and again, eventually adds up to more than the tool would have cost to simply buy from the start, and past that point, continuing to rent is the more expensive choice even though renting looked like the obviously cheaper one at the very beginning. Sheet metal fabrication is the rental option in this comparison, cheap to start, expensive per unit repeated indefinitely, while a tooled process like progressive die stamping is the purchase, expensive to start, cheap per unit once the tool actually exists, and which of the two is actually the better deal depends entirely on how many times the job is going to be repeated.
Where the crossover point actually sits
A progressive stamping die, built specifically to punch and form one exact part shape in a single automated stroke, commonly costs a great deal more to design and build than a comparable sheet metal cutting programme ever does, sometimes running to many times the cost of simply cutting the first hundred parts by laser and bending them by hand. What that stamping die buys in return is speed and a near-total absence of manual labour per part once it is running, each stroke of the press producing a finished part in a fraction of a second where the sheet metal route still needs a real operator loading, cutting, unloading and bending every individual piece by hand. At low quantities, the stamping die's enormous upfront cost dwarfs any labour saving it could possibly offer, and sheet metal wins comfortably. At high enough quantities, the accumulated labour cost of cutting and bending every part individually eventually overtakes the stamping die's fixed cost, and the balance tips the other way, the exact quantity at which that happens depending on the part's own complexity and the specific labour and tooling costs involved. A simple part with only one or two straightforward bends pushes that crossover quantity much higher, since the sheet metal route stays quick and cheap per part for a very long time before the labour cost genuinely adds up, while a complex part with many bends, holes and features close to their own minimum clearances pulls the crossover quantity down considerably, since every one of those extra manual steps adds its own share of labour cost to each individual part the sheet metal route produces.
One figure worth keeping in mind
A progressive stamping die can cost many times more to build than an equivalent sheet metal cutting and bending programme, but because that cost is paid once and then spread across every part the die ever stamps, a production run climbing into the tens of thousands of units can leave the stamped part meaningfully cheaper per piece than the sheet metal equivalent, purely because the sheet metal route never stops paying real labour and machine time on every single additional unit.
Where this stops being true
None of this crossover applies automatically to every part, since a genuinely large or unusually shaped part can be difficult or prohibitively expensive to tool for stamping regardless of quantity, and a part expected to change design more than once during its production life loses a great deal of a stamping die's advantage, since a modified die can cost nearly as much as the original one did while a sheet metal cutting programme simply gets edited and re-run at no meaningful extra cost at all. The honest comparison always has to weigh the specific part's size, complexity and expected total quantity against both routes directly, rather than assuming either process is automatically the cheaper one regardless of how many units the job actually needs.