What is a good / average price for a bag of Vermiculite or Perlite?

$15 - $20 for a 4 cubic foot bag is a decent / average price.

You can also buy it online at Uline.com. They have a 2 bag minimum purchase, but their price / shipping is decent -

While 4 Grades are available (from Course to Super Fine), you want to purchase a Grade 2 or 3.