
The Nexus 5X and 6P is compatible with Google's Project Fi's wireless service.
In areas where there is no Wi-Fi service, the SIM card in the devices will decide whether T-Mobile or Sprint is offering the better service at the moment, and connects to that carrier. In addition, Project Fi customers will have the option to finance the purchase of a Nexus 5X or Nexus 6P over 24 months. That would make the cheapest Nexus 5X $15.79 a month, while the most expensive Nexus 6P would run $27.04 a month. Google does do a credit check, and will allow you to pay off your phone earlier if you so desire.
Source: Android Central