Re: Should I feel guilty? Am I taking advantage?
Someone could still sign up today and take the class. It isn't your fault no one else signed up. If you decided to have her drive to you since you're the only couple - then yes pay extra. But if you are driving to her class, at her time and location of choice then pay for the price of that class and nothing more.
I would thank her gushingly perhaps at the first class for not cancelling but that is the most I'd do.