Fullness99 via eBay offers the Tire Pressure Monitor Valve Stem Cap 4-Pack for $1.98 with free shipping. That's tied with our mention from last week as the lowest total price we've seen. (It's the best we could find now by a buck, outside of other eBay merchants.) These caps fit on any standard size tire valve stem and provide visual alerts of each tire's pressure.
Note: This ships from China with an estimated delivery time of four to eight weeks.