Cafago offers the KKmoon Mini Professional 3-in-1 Water Quality Tester for $24.98. Coupon code "E127S" cuts that to $19.99. With free shipping, that's the lowest price we could find by $13. It features an LED-backlit LCD and measures the pH, TDS, and temperature of water.

Note: The countdown timer on the product page does not reflect an actual expiration time and resets daily. Also, this item ships from China and may take up to four weeks to arrive.