There are two models, both exclusively available from Amazon. The less expensive one is $44 and the more expensive one is $50. What you get for $6 is a jack on the device that allows you to connect it to a landline for calls (in addition to those made via the Obihai device). If you have another line this is one more way to get 911 service. You can check it out on their website.
At this time it is free to the US and Canada (through 2011). Google may or may not extend that deadline. Also, it doesn't have to work with GV, you can use it with other services too. And mind you that this isn't a formal agreement to use GV with the Obihai - GV could change the API at any time.