convert_free.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function convert_zero_to_free( $amount, $return_raw ) {
    if ( ! $return_raw || ! is_admin() ) {
     if ( $amount == 0 && is_singular('espresso_events')) {
      add_filter(
        'FHEE__EE_Ticket_Selector__display_ticket_selector_submit__btn_text',
        function(){return 'RSVP now';}
      );
     }
     $amount = $amount == 0 ?  'No Cost' : $amount; 
    }
    return $amount;
}
add_filter( 'FHEE__EEH_Template__format_currency__amount', 'convert_zero_to_free', 10, 2 );