With orders of $25 or more, Amazon offers Prime members Kong Premium Catnip 2-oz. Jar for $4.99 with free shipping. (Non-members can spend $49 to bag free shipping.) That's the lowest total price we could find by $3. It's natural mountain-grown catnip.
Note that this item is part of Amazon's "Add-on program" and can only be purchased with an order of $25 or more.