Amazon has the 12-Count Vacplus Automatic Toilet Bowl Cleaner Tablets on sale for $9.99 - 50% off coupon (check "Save an extra" box, eligible on first S&S order for this product) - 5% off with Subscribe & Save (can cancel at any time) = $4.49 with free shipping on $35+ or with Prime. Each tablet lasts at least 15 days. 37 cents per tablet. 

• Also on sale: 30-Pack for $8.99 after coupon, S&S. 30 cents per tablet.