I just looked at the AT&T plans and it appears that two lines with unlimited data would cost $120 per month (before they add on who knows what additional fees that aren't included in that price). The price for more lines goes up from there.
That's total, right? That's still only $60/mo, and it goes down with more people. Most big carriers are like $45 to $50 after 4 or 5 people, which is similar to what Fi costs.
But yeah the misc fees are annoying, especially if you ever travel or call overseas.
Edit: Wait, are you implying that Mint's pricing is $35 total, not per line?? No matter how many lines?