Displaying Price Based on Selection

I have a very simple requirement to display a price to the user based on selection of a sponsorship level as follows:

Software: $2500
Gold: $1000
Silver: $500

I believe I need to create a JS that hides a form element until a sponsorship level is selected, then determine and display the amount. I can figure out the JS for this, the question is how do I output the value to the element on the form?

I finally got this to work, albeit not the cleanest code. For some reason I could not get the switch statement to work for the three cases.

window.addEvent('domready', function() {
  var sponsorship_amount;
  $('level').addEvent('change', function() {
    if ($('level').value == 'Software') {
      sponsorship_amount = 2500;
    } else if ($('level').value == 'Gold') {
      sponsorship_amount = 1000;
    } else if ($('level').value == 'Silver') {
      sponsorship_amount = 500;
    } else {
      sponsorship_amount = "";
    }
    $('total_amount').innerHTML = sponsorship_amount;
  });
});