Get cost description from block based event.

example.php

<?php
/**
 * Get event cost description.
 *
 * @param $post_id - post id or post object. 
 * @return string
 */
function get_cost_description( int $post_id = 0 ) {

	$post = get_post( $post_id );

	if ( empty( $post->post_content ) ) {
		return '';
	}

	$blocks = parse_blocks( $post->post_content );
	$block_description = '';
	foreach( $blocks as $block ) {
		if( 'tribe/event-price' === $block['blockName'] ) {
			if ( isset( $block['attrs']['costDescription'] ) ) {
				$block_description = $block['attrs']['costDescription'];
			}
			break;
		}
	}

	return $block_description;
}