Fullness99 via eBay offers this Tire Pressure Indicator Valve Stem Caps 4-Pack
with free shipping
. That's tied with our mention from a week ago as the lowest total price we've seen. (It's a current low by a buck, excluding other eBay vendors.) These caps fit on any standard size tire valve stem and provide visual alerts of each tire's pressure.
Note: This item ships from mainland China and may take three to eight weeks to arrive.