Not quite. They have a soft 15 Gb cap, past which they throttle you down significantly but also don't charge you. And then it's $10 / Gb if you want to extend that cap.
There's more than one plan. The one I'm on is $10/GB until you use 6 GB (or $60). You're right though that they slow down after 15 GB (not Gb) but I've never come anywhere near that.
The way I look at it is that my plan is $20 then my data is $1 / 100 MB.
The way I look at it is that my plan is $20 then my data is $1 / 100 MB.