Like everything else, it depends. Just like buying a car. I have the Ipod/Itouch with 32kg.........$310................You can get an ITouch for around $200 and 8kg. I'm sure there are some that are cheaper. Received mine as a gift for Xmas so wasn't out too much ;-)
Once you download this app, you do not have to be connected to the Internet for it's features. Any updates that you wish to have added, of course would require that service. Which you can easily acquire if you have Internet access, which you do.