When creating/editing a product, you  have the ability to set the MSRP (retail price) on the item. The MSRP itself is never shown to clients, however, it is used to calculate the discount which can be displayed to your clients on proposals and invoices. 

Note: The discount off of MSRP feature is an optional feature and by default, the discount will not be shown to the customer until specifically enabled.

So, how do you set the discount? When creating a proposal or invoice,  click on the “MSRP discount” link, this will open up a popup that shows the MSRP as set up on the product.  If you didn't set up the MSRP on the product, or need to edit it, you can easily do so within the popup.  Once you set the MSRP per unit, click on the "Calculate discount" button, this will automatically calculate the discount off MSRP and display that to clients within the proposal/invoice.
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To hide the MSRP discount from a client (after selecting to display) access the edit mode of your proposal or invoice, locate the relevant product line item, and click the little "x" next to the calculated percentage.