What I'm doing wouldn't be cost effective for making ingots when it comes to the hot stuff.
Large amounts of lead? ... sure, that's fairly cold ... melting scrap would need 900F or so, or a little less than 700 for clean lead (if I remember correctly pure lead melts at 620 or so (alloys are almost always at lower temp) ... so, a good propane fish cooker and a cast iron pot (that you never use for anything else ever again), regular old wax for flux, and molds of cast iron or aluminum.
For the hotter stuff, never actually measured temperature. I've mostly done small stuff ... like sometimes just a ladle and acetylene torch.
For big hot stuff : Fuel is a choice between money and labor. You can make your own charcoal and bellows for cheap and a lot of work (both building and working) , graduate to using dried air out of a compressor, or all the way up to expensive gasses and oxygen. No matter the fuel, a blast furnace is needed, but easy to make. Mine is currently a stack of bricks, asbestos pipe, and steel pipe in the corner of the basement. I set it up in the back yard when needed.
Cost in time/work/fuel isn't worth the difference in price at the scrap yard to me. Doing it on a large enough scale to be "worth it" would also make it hard to control the pollution it can generate. Casting art pieces, you could make good money, but "art" is all about marketing rather than objective value.
