The easiest way to do this is to use the 'Cost Price' field on the 'General' pane of the product to the far right of the normal 'Price' field.

What you will need to do is add the sale price to the normal 'Price' field and add the correct price to the 'Cost Price' field.

You will then need to add some code to the product layout above the price in the design for this to work. Add the following code: -

<actinic:block if="%3cactinic%3avariable%20name%3d%22ProductCostPriceFormatted%22%20%2f%3e%20%21%3d%20%22%a30%2e00%22" >
Normal Price: <actinic:variable name="ProductCostPriceFormatted" />

Into the product layout.

Go to the 'Design' tab and then go to 'Windown | Restore Split View Layout' and then in the top viewing pane click on the description of the product. This will automatically display the 'ProductDescription' variable and should look like this or similar: -

<p class="product-text">

add a new line below the </p> and add the new code above then click the button 'Apply changes'.

Once you have done this upload the site for the changes to take place.

Now what will happen is that every product with something other than 0.00 entered into the 'Cost Price' field will display: -

Normal Price: <price of product>

(with the price replacing the text <price of product>) just below the description field.