Source: http://arstechnica.com/gadgets/2015/04/h...

It's hard to label any one service as "cheaper" than another—it really comes down to what fits your usage patterns. Project Fi's pay-only-for-the-data-you-use policy has the potential for savings, but if you use roughly the same amount of data per month and can perfectly fit yourself into a plan from another prepaid carrier, you have a good chance of saving money elsewhere.

Project Fi is great for people with fluctuating data usage though. Take me for instance: most days, at home and at work, I'm on Wi-Fi, with barely any data usage, but there are those months where I travel a lot, and then my data usage spikes. Project Fi would give me money back for the low-data months, while flexing to a larger plan when during busy months. For a person like me, it's perfect. I don't need data all the time, but when I do need it, I need it to be fast and plentiful.

Still, price isn't the only thing to consider when choosing a network. Fi also has a lot of great features that don't show up in the text/talk/data breakdown. Project Fi's combo Sprint and T-Mobile plan should have better coverage than either Sprint or T-Mobile individually, and the 3G international data will be a big deal for some people. You're also getting the ability to send calls and texts to all your devices—it basically has a (hopefully) non-terrible version of Google Voice built in.