If you wanted to get this incorporated with all of the eCart features and functionality available when doing so you can think of your sign up form as the product. You would just apply an add to cart to this page and set the price. Then you would just redirect the user straight to the checkout page from here.
Doing it this way you can still make use of eCart's interfaces for the checkout wizard and other parts. If you have any questions about any of the specifics please post back and we would be glad to help.