Amazon offers Prime members the Polar WearLink + Transmitter Nike+ Heart Rate Monitor for $14.99 with free shipping. That's the lowest price we could find by $5 and the best deal we've seen. It provides heart rate information to the web service via compatible Nike+ devices. It features 5kHz wireless transmission, fits chest sizes from 26" to 60", and up to a 600-hour battery life.

Update: The price has dropped to $14.11.