The villager trade is 20-22 emeralds per. If each emerald is 30, thats 600 at the lowest end. I'd say 750 is a good, sustainable price. I sell mine for 900 so I can keep some stock. Some people do sell them for 500 or even less but it's rare they keep stock and should really just be selling their emeralds instead at that point.
And it's actually a price which holds true for most of the SMPs too. Some prices differ quite heavily but this seems to be quite equal.