So, I'm putting my welder hat on for a minute... there is no cheap 304. What they use is 201 SS, which has some of the properties of the 300 series of stainless, but made with cheaper manganese instead of nickel. Before I got certified as a SS welder I thought the same as you. Almost all of the cheap exhaust sold on ebay as 304 stainless, its just 201. I was sold some welding rods for TIG that were supposed to be 304, but ended up being 201, they behave entirely different when welding. An expensive lesson learned.
Rolled Metal Products is one of the steel industry’s most respected service centers with facilities across North America. We stock Stainless Steel, Nickel Alloys, Aluminum and other Specialty Metals in coil form and also offer toll processing of various metals including: Stainless Steel, Nickel...
rolledmetalproducts.com
Not exactly, 304 stainless is 18/10. 18/8 is the family of the 300 series stainless, which means they have no less then 18% Cr and 8% Ni, there are a few websites that erroneously say that 304 has "no more" than 8%, but the AISI standard for 304 is between 8-10.5% Ni.
Any true 300 series will not rust, 201 shouldn't rust either but will tarnish. 400 series will develop surface rust, but it will not penetrate the metal.
Welders hat off.