Filter pmpro_has_membership_access so paid members have access to all addon packages, but free members must pay the per post price. Edit the $my_paid_level_ids array to include an array of your paid level ids. Put this code in your active theme's functions.php or a custom WordPress plugin.

my_pmpro_has_membership_access_filter.php

<?php
/*
    Filter pmpro_has_membership_access based on paid membership access.
*/
function my_pmpro_has_membership_access_filter( $hasaccess, $mypost, $myuser, $post_membership_levels ) {
	$my_paid_level_ids = array(5,6,7);

	// Check if the user doesn't have access
	if( ! $hasaccess ) {
		// If this post has membership levels associated with it and is supposed to be locked by the Addon Plugin
		if ( ! empty( $post_membership_levels ) && pmproap_isPostLocked( $mypost->ID ) ) {
			// Loop through post levels
			foreach ( $post_membership_levels as $level ) {
				// Change level ID here to match your paying membership level
				if ( in_array($level->id, $my_paid_level_ids) && pmpro_hasMembershipLevel( $my_paid_level_ids, $myuser->ID ) ) {
					$hasaccess = true;
				}
			}
		}
	}

	return $hasaccess;
}
// Hook in after the PMPro Addon Plugin
add_filter( 'pmpro_has_membership_access_filter', 'my_pmpro_has_membership_access_filter', 20, 4 );
?>