If you already have a Sprint unlimited calling plan (whether it's a voice-only plan or part of a multi-service package, such as Simply Everything), you pay a premium of $5 per month to extend that coverage into your home, plus the one-time $100 fee for the device. If you have no existing Sprint package and are seeking cheap home voice-calling service (perhaps in lieu of a landline), you have the one-time $100 device fee, plus the $5-a-month premium plus $10 a month unlimited-from-home calling plan (or $20/month if you want your whole family to share the service). In other words, the femtocell in-home voice-only fee would be $100 (one-time) plus $15 or $25 per month. If you want calling service, data, etc., outside the home, you need a different plan.

P.S. GPS isn't known for working well indoors, so the W911 concern is a legitimate one, and one that E911 VoIP requirements in certain states and industries have been up against for a while.