Amazon offers its Prime members the Bell Automotive Tire Pressure Monitoring 32 PSI Valve Caps for $8.98 with free shipping. (Non-Prime members can spend $35 or more to qualify for free shipping; otherwise, shipping adds $5.58.) Aside from the mention below, that's the lowest total price we could find by $6. These caps allow you to check tire pressure without removing them.

A close price: Groupon has these valve caps for $5.99 plus $3.99 for shipping.

Update: Expect shipping to now take two to five weeks at Amazon.