Form Process Question

I have a client that I have made some forms for. On one of the forms, their customers are to registar for a class. The cost of the class is 195 per person. She would like to have the people be able to have the total cost calculated for them before they submit the form. (I already have the cost calculated in the email and result page). I thought about creating a separate form within the original form page that could calculate the cost, and process it to itself, I suppose I would need to create a session so all the information in the first form is not wiped out. Is there an easier way to do this or is there a way to create this calculator that changes upon the input instead of clicking a calculate button? Thanks Gary