Steps to achieve the same:
1. Have to write a script in a JS file which will handle the on change event for the drop down (the source element)
2. In script - make Ajax call to retrieve the price based on the value of selected element.
3. Write a action in controller to retrieve and send back the price / output to the request
4. In the success function for the ajax call - set the value of the field with the element retrieved from the ajax request.
1 more important info - add the js in action using set_js function ...
and in the js - call for window.onload function and not $.ready - the $.ready will fail...
That's the way u do it..