B&H Photo Video offers the Polar WearLink + Transmitter Nike+ Heart Rate Monitor
, model no. 92043571, for $24.95
with free shipping
charges the same for Prime members.) That's the lowest total price we could find by $19. It provides heart rate information to the nikeplus.com 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.