Is it possible to add custom business logic to a Custom Post Admin Edit Page?

You can create a plugin to add JavaScript (and stylesheets) in targeted admin pages. By the description, I think the better is to inject the total amount field together with the existent fields. Or you can create a meta box to display it. And use Stack Overflow for all your jQuery needs.

<?php
/* Plugin Name: My plugin */

# Run only in /wp-admin/post.php
add_action( 'admin_print_scripts-post.php', function()
{
    global $typenow;

    // Run only for the types Posts and Movies
    if( !in_array( $typenow, array( 'post', 'movie' ) ) )
        return;

    # codex.wordpress.org/Function_Reference/wp_enqueue_script
    wp_enqueue_script( 
            'my-custom-posts', 
            plugins_url( '/my-custom-posts.js', __FILE__ ), 
            array(), // dependencies
            false, // version
            true // on footer
    );
    # codex.wordpress.org/Function_Reference/wp_localize_script
    wp_localize_script( 
        'my-custom-posts', 
        'my_vars',
        array( 'typenow' => $typenow ) 
    );
});

And the file my-custom-posts.js in the same folder as your plugin:

jQuery(document).ready(function($) 
{    
    alert( my_vars.typenow );
    // do your stuff
});