Google charges $10 per 1GB of data, but only bills customers for what they actually use each month. The offer is available only to existing Project Fi subscribers. Tablets cannot be used as mobile hotspots.
The initial batch of tablets compatible with the service include the Nexus 7 and Nexus 9, iPad Air 2 and iPad Mini 4, and the Galaxy Tab S. Project Fi uses a mix of cellular service from Sprint and T-Mobile, and WiFi to provide coverage. Devices that run on Project Fi can dynamically switch to the strongest network to optimize performance.
Source: Project Fi Help Forum