Change field value with Ajax call based on checkbox state

Change field value with Ajax call based on checkbox state

I've created a registration form with sponsorship options hooked to checkboxes.  My original form used ASP to keep values in hidden fields to use for totaling everything at checkout time.

Now I'd like to use PHP and Ajax to place the value into the text field related for each option checked.

I'm able to do that making an Ajax call.

My problem that I need help with is this:

If the visitor decides to uncheck a sponsorship option, the value placed in the associated field remains on the form.  How do I replace that value with a null or empty string if they decide to uncheck their original choice?

I've tried JavaScript to no avail so far, both in the client side and server side code.  I'm very new at this and I need help.  Will jQuery work without having to change a bunch of code? 

Thanks for any help you can provide to a neophyte.