People with inadequate internet service (e.g. jet pack) that could also be metered service, and buy an Echo are jumping the gun. They should beef up their internet first.
Before I tell you how much bandwidth is being used by the Echo when it streams a TuneIn station, are you sitting down? OK, according to the Traffic Monitor of my router, streaming the station was using around 40 KB/sec continuously until I turned off station stream. If I did the math correctly, that equates to 144MB used for one hour of streaming. You can do the math for the number of hours you would stream each day.