I think that's not quite how Fi works.
The talk and text part is $20 a month (prorated by days if you cancel half way), then data is $10 a gig and you are charged only what you use. So if you are at home all the time on wifi, and use only 10MB, that month your data is 10c plus $20 = $20.10.
The main benefit on Fi is international coverage for the same price. This really helps on trips and vacations. I get a new line every trip and then cancel it afterward. Ends up with less than $20 for a couple weeks roaming the world.
Ever since 4G LTE everyone has sim card and unlocked phones. You DO NOT have to buy a Verizon phone to use it on Verizon, many phones have coverage on Verizon band despite from ATT, Sprint, TMo, etc. Just look for the bands, assuming you are buying a legit big name phone and not some "free" low end phone that company gave out.