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With the latest versions of the Debut theme (17.13.* at the moment of writing these lines), you don't need to touch the source code to display the 'compare at price'.
To display the price 'compare_at_price', you just have to enter the price 'compare at price' when creating the variant or modify it after its creation.
"Compare At Price" simply means the price it would originally sell for. So if you're selling it for $25 on sale, and it's typically $30, you would put the $30 as the "Compare At Price".
Let us say that your product X has a standard retail price of $50 (put that as compared to), and now discount it by marking it at 39.99 - this way your shoppers know that the average price for item x is 10 dollars more expensive than what you have it listed for.