Going back to the original question -- I got a long and very technical explanation from Tobin George of Deep Sea Supply once, that had to do with the specific alloys used in various grades of stainless, and how some of them are more corrosion-resistant in salt water than others, and how the ones that are, are expensive.
There is also a difference in finish quality. Some plates are thoroughly deburred and others aren't. DSS plates have elastomeric grommets in the lower slots that obviate the necessity for triglides, and make adjustment a bit easier. Some plates don't have camband slots (the Halcyon small pattern plate, for example) and some have fewer bolt holes/slots than others.
There is also a difference in finish quality. Some plates are thoroughly deburred and others aren't. DSS plates have elastomeric grommets in the lower slots that obviate the necessity for triglides, and make adjustment a bit easier. Some plates don't have camband slots (the Halcyon small pattern plate, for example) and some have fewer bolt holes/slots than others.