You can get to around 200C with oil-based media. But oil goes bad pretty quickly, and it leaches tons of crap out of everything.

Above that temperature, you need molten salts or liquid metals that are extremely corrosive. Your other option is to use gaseous media instead.

Yeah good point. I think the point about cost is the real crux here, regardless of working temp. (CSP has used molten salt but it is quite expensive to store and maintain.)