|The Finger Sensor (uses the headphone jack)|
HRV measures the variability of your heart rate while breathing at a slow, even pace. While I am not clear on each factor in the measurement, essentially by using a finger sensor or chest strap and breathing at a selected pace for around a minute, HRV is established as the sensor looks for changes in beat to beat timing. A number is established and after around a week a baseline is established. The best time to collect data is when you wake up, thus you have a consistent measurement.
So what is the cost? While more than one place exists, I use iThlete. You will have to make two purchases in the matter, the app which costs around $8.00, and your choice of a sensor, which will set you back $60-$70. Either way, the data collection is immense, especially as you rate the various factors in your life that contributed to the rise and fall of your HRV, from training load to sleep to mood to diet to stress. While it may not be for everyone, it can lead to a healthier training pattern. If you have questions, ask away.