Hi,
$3 for each half hour.
I wouldn't say 'each half an hour' is wrong here, but I don't hear it said much.
I waited for half an hour.
I waited for a half hour.
By the way, how's your garage door?
It has its ups and downs. (This is a joke.

Do you get it?)
I'd like to know what country you are in, please. I want to know where my words are going at midnight, my time.
Clive