It's not all that cheap if you're a particularly heavy user. It's cheaper than the major Telecom companies (AT&T, Verizon, Sprint etc.) if you keep it under4-6 GBs, but you can get 6 GBs on Tmobile's data only plan and simply use Google's voice service with it.
Furthermore, T-mobile offers unlimited streaming from almost every major streaming service (Netflix, YouTube, Spotify, Hulu, etc.) which is WAY more bang for your buck than what Google is offering as streaming is likely 90% of your data usage if you use it liberally.
Of course, this is also a blatant violation of net neutrality principles.