The Events Calendar CSV import: Find matching event solely by a custom field. Replace the function match_existing_post() in src\Tribe\Importer\File_Importer_Events.php. In the importer you need to select the column to match by as 'Cost'.

the-events-calendar-import-match-custom-field.php

<?php
	protected function match_existing_post( array $record ) {

		global $post;
		$import_id = $this->get_value_by_key( $record, 'event_cost' );
		if ( ! empty( $import_id ) && is_numeric( $import_id ) ) {
			$args = array(
				'post_status' => 'any',
				'post_type' => 'any', // circumvent tribe_events filtering of past events
				'posts_per_page' => 1,
					'meta_query' => array(
					array(
						'key' => 'ek_import_id',
						'value' => intval( $import_id ),
					)
				)
			);

			$get_posts = new WP_Query();
			$get_posts->query($args);
			$matches = array();
			if( $get_posts->have_posts() ) :
				while($get_posts->have_posts()) :
					$get_posts->the_post();

					$matches[] = get_the_ID();

				endwhile;
			endif;

			// Restore original Post Data
			wp_reset_postdata();
		}



		if ( empty( $matches ) ) {
			return 0;
		}

		return reset( $matches );
	}