metered bandwidth doesn't actually make 1 to 1 sense with the cost increases, it just works in the real world as a disincentive for use.
Unlimited internet connections were a great marketing item when there was no readily accessible way for users to saturate their home connections for more than a minute or two.
The whole oversell design for ISPs depends on the concept that only a fraction of your userbase will be using the full bandwidth available to them at any given time. Unfortunately with the rise of streaming services that's no longer the case and so now the oversell ratios that used to be great for ISPs are no longer working.
There are 2 ways to solve this:
1. Build bigger pipes: This is expensive and unless you're building enough capacity so that every user can use their entire connected speed it's not going to be a full solution. It gets you part of the way there though. Unfortunately to do this you'd have to pass heavy costs along to the consumer, this probably won't work as long as the other ISPs are willing to sell the same customer a connection for $30-50/month
2. Get people to use less of the pipe: Go ahead, ask everyone nicely to use less internet, to refrain from streaming netflix, hulu, itunes movies etc.. Good luck with that. The only way to get people to use less bandwidth is to charge for it and make them not want to use it. The goal of these pricing plans isn't to earn money for the bandwidth used, it's to encourage the bandwidth hog users to move to another ISP, thereby making them Comcast or AT&T's problem.