Project Fi now has "bill protection" which caps your bill at $80 if you use more than 6GB in a month and you'll continue to get high speed data up until the 15GB mark. At that point, you will be slowed down to 256Kbps but you are given the option to pay $10/GB for more high speed data. If you use less than 6GB, you'll pay the normal $10/GB price.
This addresses one of the pain points about Project Fi which is that it loses value very quickly if you use more data. While it's still more expensive than prepaid options like MetroPCS or Boost Mobile, it offers international talk, text, and 256kbps data internationally at $10/GB and runs on 3 different networks domestically so it might be worth it.