gistfile1.php

<?php

// somewhere in single.php
if( time_is_paid_content() ) {

	if( time_is_paid_user() ) {
		the_content();
	} else {
		echo 'Sorry, you must log in or sign up to view this content.'
	}

} else {
	the_content();
}

// somewhere else
function time_is_paid_content() {
	// checks post meta to see if content is paid
}

function time_is_paid_user() {

	$validate_user = function() {
		// validates the users cookie, returns bool
	};
	
	if( function_exists( 'vary_cache_on_function' ) ) {
		// im probably using this anonymous function wrong but you get the idea
		vary_cache_on_function( "return $validate_user" );
	}

	return $validate_user;
}