I feel like every two years I need to call my carrier and complain if I want a decent deal. They will do things like upgrade my plan on their website to have 10 extra gigabytes of data but won't upgrade me to it until I contact them. There's also all the new member exclusive deals that I feel make it impractical to just sit on one plan for an extended period of time.
I pay $15 a month for a plan with 5gb data. But even their unlimited plan is $30 a month. So yes, mint is cheaper assuming you are talking about just one phone.