As the others have stated, take a look at what you need to cover your costs, what you want to net and then set a price. You can also see what your competition is offering for some ideas on pricing for your area and package ideas.
You may also want to consider adding a “unlimited” monthly package to your punch card. You can keep the punch card and add a “unlimited” pass to for the month. Clients can attend as many classes as they’d want and they would get billed monthly through an automatic ETF. They sign a contract allowing this and notifiy you in advance when they want to cancel, usually a two week notice.
I was surprised that you state the fact that you begin to use Quickbooks as a rationale for changing your pricing structure. A program should never determine the way you do business, and the pricing is an entirely different entity.
What you charge is dependent on your market. I am aware of the pricing structure of some clubs here in Raleigh, NC. It is Buy 10, get one free, at a rate of $10 per class.