Tough question, too many variables.
Let's make some assumptions:
1. You are buying hot rolled plate
2. in standard sizes
3. in large quantities (at least 50T)
There are two componants to the price, base and surcharge.
304, 304L, 304H and 321 all will have about the same surcharge.
The current H premium is small, usually less than 5%
321 carries a larger premium, prob about 20%
309 and 310 are another issue. This is where it is very important to ask about surcharges. I have included them in these numbers.
309 - roughly 1.5 to 1.8 times the 304L price.
310 - with its higher Ni is at least 2.0 to 2.6 times hte 304L price.
But if you are looking at a high temp application the 309/310 are probably still the better value. Don't let up front cost sway you too much. Build a simple life cycle cost model. Consider what maintance, down time and replacement could cost you in the future.
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Corrosion never sleeps, but it can be managed.