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