Yes, it would cost a lot to do prototypes of silicon steel parts.
On a former job, for proto laminations for motors, we used electro-erosion on complete lamination packets for creating a shape in the full packet. Of course, you can't alternate interlaced shapes (not needed usually for a motor), but you can do the same with a transformer, the alternating is a small optimization you could skip.
I don't recommend the electro-erosion because it'S expensive.
Also, of course, the HF (100kHz) SMPS version is much more compact, less cost, and higher efficiency, but it's a hassle to develop, and not worth it for only a few pieces compared to the 50Hz bulky transformer variant.