The Valkyrie was the tube-chassis, V8 version of the Avenger. The Avenger was based on a VW chassis, with VW mechanicals.
The Valkyrie is worth considerably more than $500; that's the going price for a rough Avenger.
A good place to find out some more info on the Avenger or the Valkyrie is:
http://www.avenger-valkyrie.org/
Or you can email me.
Your pal,
Meat.