It’s not as complicated as some make out, is it? That’s anywhere between 75 and 150 miles in 20 minutes, depending on weather/driving style, and that really can’t be bad.
Your maths makes a lot more sense that the actual cost unless there’s a connection charge, which is incredibly rare these days.
I'm Autistic, if I say something you find offensive, please let me know, I can guarantee it was unintentional.
I'll try to give my honest opinion but am always open to learning.
Mark