Price / discount overlays are pretty simple to set up.
At the ad level, scroll to the Show More Catalog Information section, and click to check the box next to Add an overlay to images (see below):
Once you click on the dropdown menu, you can choose whether to show the actual price, the strikethrough price or percentage off in your product catalog images. To change the appearance of your overlay, click Edit appearance.
Remember, you can choose the shape, font, color and overall position of the overlay. When you finish, click Save, your overlay is now ready, and you can finish creating your dynamic ad.