But copper is not cheap though
bd139: Well... that was just statement of required amount of wire being much larger for air coils
I have no problem accessing a lot of different enamelled wires, including TEX wire. I work with power conversion and transformers a lot.
phenol: Curiosity is always welcome! Nothing exact was on my mind when I was asking. Sometimes I feel a need to build some radio prototype of some sorts, sometimes working with peculiar radio ICs that require transformers for operation.. so why not get the proper ones, but in more modern shape and size. I have only available those very old ones, which are kind of big (on the other hand very easy to rewind and modify).
I have briefly browsed on Aliexpress for RF toroidal cores. Definitely there are lots of them available. Would not classify them exactly as cheap, compared to the standard mix #s used in power conversion like #26, #52 <--- at least those I can source for a fair price from a local distributor.
I have also a decent set of RF cores I have hoardered over the years, however the materials have very different coding style and getting proper datasheets for them is kind of complicated, as these have been only manufactured locally "back in those days". Sometimes even if you (think to) have the right datasheet, even simple measurements of the cores does not correlate to the datasheet very well. It is possible such large manufacturing deviations were present back then, but who really knows. I even don't know now or can't remember, if these are ferrites or iron powder. As our German colleagues would say: Keine Ahnung!