It's actually 15gb for $100 now. If you go the Mobile Value Share plan, it breaks down like this.
$100 --15gb data, unlimited talk and text
$15 - your phone
$15 - wife's phone
$15 - mom's phone
-------------------------
$145
But then you add on the NEXT pyts for each (depending on what phone you get. Let's just say $30 a mo. as an example.
$15, plus $30 for each line
------------------------------
$235 before fees and taxes
If you stay on your plan and sign a 2yr agreement and subsidize, you will pay $40 per line forever, even after two yrs is up.
If you move to MVP, your lines will stay at $15 even after you have paid them off. So, you need to do the math and go for the plan that works best for you.
Most people on the Unlimited data plans, buy their phones outright, so there is no change to their plans and they keep their unlimited data. If you can afford to do this, then that is the best plan.