Adding Base Cost to the Booking Price.

functions.php

<?php

// Display Fields
function woocommerce_product_base_cost_fields() {

	global $woocommerce, $post;
	echo '<div class="product_custom_field">';

	// Custom Product Number Field.
	woocommerce_wp_text_input(
		array(
			'id'                => '_bkap_booking_base_cost',
			'placeholder'       => '0.00',
			'label'             => __( 'Base Cost', 'woocommerce' ),
			'type'              => 'number',
			'custom_attributes' => array(
				'step' => 'any',
				'min'  => '0'
			),
		)
	);

	echo '</div>';
}
add_action( 'woocommerce_product_options_general_product_data', 'woocommerce_product_base_cost_fields' );

// Save Fields.
function woocommerce_product_base_cost_fields_save( $post_id ) {

	// Custom Product Number Field.
	$woocommerce_custom_product_number_field = $_POST['_bkap_booking_base_cost'];
	if ( ! empty( $woocommerce_custom_product_number_field ) ) {
		update_post_meta( $post_id, '_bkap_booking_base_cost', esc_attr( $woocommerce_custom_product_number_field ) );
	}
}
add_action( 'woocommerce_process_product_meta', 'woocommerce_product_base_cost_fields_save', 10, 1 );

// Adding Base Cost to Booking Price.
function bkap_booking_price_with_base_cost( $price, $product_id ) {

	$base_cost_value = get_post_meta( $product_id, '_bkap_booking_base_cost', true );
	if ( isset( $base_cost_value ) ) {
		$price = $price + (float) $base_cost_value;
	}
	return $price;
}
add_filter( 'bkap_modify_booking_price', 'bkap_booking_price_with_base_cost', 10, 2 );