This recipe changes the level cost text "Free" for free levels using the gettext filter.

change-text-free-level-cost-text.php

<?php
/**
 * This recipe changes the level cost text "Free" for
 * free levels using the gettext filter.
 *
 * @link http://codex.wordpress.org/Plugin_API/Filter_Reference/gettext
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function change_free_level_cost_example( $translated_text, $text, $domain ) {
	if ( 'paid-memberships-pro' !== $domain ) {
		return $translated_text;
	}

	switch ( $text ) {
		case 'Free':
			$translated_text = __( '$0' );
			break;
	}

	return $translated_text;
}
add_filter( 'gettext', 'change_free_level_cost_example', 30, 3 );