Why the Trident VGA graphics cards are so expensive if their production finished almost 30 years ago?

People like to repair old computers to play retro games not available on modern systems and they need parts.  Since they are no longer manufactured the supply that remains gets priced higher. 

because almost all of them have long since been junked. so there's a shortage

The average price should be in the child's pocket money range. Any thing outside that range are scammers looking for a victim.