Expenses tracking including income and total amount remaining

I am a new Tap Form user and am trying to create a form to track my Gym Class with the remaining class fee expenses.
I have tried to revise the form from the Expense Template but could not successfully figure out how to add the formula field. I would like to create a form adding the field with the total prepaid fee (e.g., $1000 for 8 classes, $2000 for 10 classes) and record each class I attend (e.g., $200 per class with the date – deducting from the prepaid fee). Then, I want to show the total amount/class remaining in the result.
If anyone has a similar form to share with me would be highly appreciated!