Steam offers downloads of Barrier X for Windows for free. (iTunes offers it for the same price for iOS, and Amazon and Google Play offer it for the same price for Android.) That's $3 off list and the lowest price we could find.
iTunes Store offers the Duet Display for iPad and iPhone for $9.99. That's tied with our October mention at $10 off and the lowest price we could find. This app allows you to use your iPad or iPhone as an extra display.