If you're adding a new item, go to Items > Create Items. If you've already created the item, go to Items > Published Items or Draft Items to find the item and click on the Edit link next to it.
In the Pricing Details section, enter the item's value - it must be an actual number. This is a required field.
Scroll down to the Optional Setting section and use the "How should we display your item’s value?" to select "Display Estimated Value as Priceless." You also have the option to not display any value (select "Do not display estimated value."