I am only passing along info that I have been told by someone who works in steel sales. I assume he is telling me the truth but I would love to read what others have to say on this. After all, we all want to know how to make the perfect furnace.
On the subject of price, a mild steel furnace normally is twice as thick as a stainless furnace so I would assume it may cost more. I have a list of metal cost from a few months ago around somewhere but I can't seem to lay my hands on it.
I find it interesting that while the subject of types of metals often include corrosion they often do not include the ability of the same metal to transfer heat. For instance, 409 is twice as resistant to transferring heat as mild steel so to have a comparable furnace it must be half as thick. 304 is even more resistant so it would need to be even thinner.
I find all of this to be fascinating. I am very glad that furnace manufacturing is moving into this more complex stage of production. Gone are the days that a manufacturer can spit out a box made of almost anything and sell it as a quality product. The more testing and refining of theories of corrosion and efficiency the better.