Add estimated value for a post according to the number of words

Here’s an idea for a starting point, by looking how the word counting is done in the /wp-admin/js/post.js file:

/**
 * Testing word price calculations
 */
add_action( 'after_wp_tiny_mce', function()
{ ?><script>
    ( function( $ ) {
        $( function() {
            // Init
            var $content = $( '#content' ),
                $count = $( '#wp-word-count' ).find( '.word-count' ),
                total_price = 0,
                price_per_word = 0.02;

            // Out price updating function
            function wpse_update_price(){
                total_price = $count.html() * price_per_word;
                // Debug output
                console.log( 'Total price: ' + total_price );
            }

            // On keyup event 
            $content.on( 'input keyup', _.debounce( wpse_update_price, 1100 ) );
        } );
    } )( jQuery );
</script><?php
} );

Here we display the total price in the console log.

Hope you can adjust it further to your needs.