Amazon[amazon.com] has 3 pack of PetSafe Drinkwell Replacement Premium Charcoal Filters for $3.99 as an add-on item (with $25 order) or less with subscribe and save:
$3.79 with 5% discount
$3.39 with 15% discount
Edit: The 12 pack[amazon.com] is also available for 9.33 as an add-on or less with S&S.
This is the cheapest I've ever seen the Petsafe brand of these filters, and lowest price ever on amazon according to CCC.