<?php

// the code below goes in your functions.php file

my_custom_prepare_shop_order_object($response, $order, $request) {
  $data = $response->get_data();
  $data['regular_price'] = get_post_meta( $data['id'], '_wholesale_price', true );
  $response->set_data($data);
  return $response;
}

add_filter( 'woocommerce_rest_prepare_product_object', array( $this, 'my_custom_prepare_product_object' ), 20, 3 );
add_filter( 'woocommerce_rest_prepare_product_variation_object', array( $this, 'my_custom_prepare_product_object' ), 20, 3 );