We use our grandfathered VERIZON MIFI Plan from back in 2010 which is a 3GB DATAPLAN. I think their lowest offered dataplan now is a 5GB service. This allows up to five of our WIFI enabled devices to be used at the same time.
I don't think we have ever gone over our 3GB limit in one month with it but we do watch it like a hawk and check the dataplan status everyday before we use it...
We only use ours for checking emails, checking in with the kids, paying some bills and some surfing the internet. I think it is known that just down-streaming one movie will eat up over 1GB of data so we have never done any of that.
Roy Ken