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.