Actually it is quite inexpensive, no custom dies(204 bushing die with 6mm expander and bushing), no fire forming. You can even chamber with a 204 reamer with a 6mm pilot and cut the neck and throat seperately.6-204 sound like an expensive way to get a 6 TCU
I have a 22-204 reamer and have done a 22-204, 6mm-204 and a 25-204 and can load them all with the same bushing die, by just changing out the guts.
I do think it would be too long for a 527 action though.