This code adds the regular price next to the discounted sale price on order emails for the customers invoice. By default, only the discounted price is shown however you can also display the regular price with a strikethrough for each product subtotal like this :
The discounted sale price and crossed out regular price are shown for every item like this :
You can also use a custom field to add additional content before or after the price in the price column on order emails.
Copy and paste the PHP code to the end of your child themes functions file.
To use the value from a custom field, use the 2nd code snippet.