So about the cost of 45-70? With similar ballistics. For a second upper, could be a fun project. Look at 450 bushmaster. Couple years ago it’s wasn’t exactly popular, but now since the ruger American ranch came out in that calibre it’s actually not that hard to find.
But it seems like something you would be better off doing on your own. Now the 300win and 338 versions they teased about, those would sell.