Amazon offers the Mothers California Gold Clay Bar System, model no. 07240, for $12.93. Add it to cart via Subscribe & Save and it drops to $10.99. With free shipping, that's the lowest total price we could find by $7. It includes two clay bars, Showtime instant detailer, a microfiber towel, and purports to remove embedded grains of metal, tree sap, airborne environmental deposits, and paint overspray.
Be sure to cancel your subscription once your order arrives in order to avoid being billed for subsequent shipments.