It's Belgian pewter, looks like a Liege touch mark. 18th century.
The price seems HIGHLY unlikely to me unless the coat of arms is important. Condition is against it, even though the work is excellent.
enquiries@pewtersociety.org
Tell them you want to know about the mark and that you are NOT selling the plate but it is family heritage
I would then contact a couple of Belgian historians or website regarding the heraldry.