Seems you select a carrier so it's whatever their rates are. The Apple SIM just allows you to use the same SIM rather than swapping out SIM cards for each carrier you might want to use (more useful when traveling internationally, I imagine, than when staying in one country).
FYI on Fi there are many devices that seem to work well, even if they aren't on the official list of supported devices.
Fi also supports additional devices using a data-only SIM, which doesn't cost any extra except for the per-GB data rates. No $15 surcharge just for the ability to use other devices, etc. Same thing with tethering.
edit: also, this isn't exactly new, Google Fi was introduced in October 2016. I've been using it since then with no problems. In fact, I once received a reimbursement for a short outage that I hadn't even realized happened.
There's no extra charge for a data-only SIM but you are required to have a primary device which costs $20 (plus $10/GB). Would be great to offer this to customers on other carriers.
It's pretty cool. Google does not charge for the SIM so I ordered one. I think I'll pick up a cheap device sometime to keep on the boat for reading and use as a backup GPS!
I've been using Fi for a while but I didn't realize they had these data-only sims available.