change_text_price.php

<?php

add_filter( 'gettext', 'change_my_cost_example', 20, 3 );
/**
 * @link http://codex.wordpress.org/Plugin_API/Filter_Reference/gettext
 */
function change_my_cost_example( $translated_text, $text, $domain ) {  

        switch ( $translated_text ) {

            case 'The price for membership is <strong>%s</strong> now' :

                $translated_text = __( 'The price for membership is <strong>%s</strong> one time', 'paid-memberships-pro' );
                break;
      
        }

    return $translated_text;
}