'iDisplay' has dropped to $2.49 on the iOS App Store it was $12.99.
Description:
Turn your iPad, iPhone or iPod touch into additional display for your Mac or Windows computer. Increase your personal productivity by extending your computer screen! Perfect tool for multitaskers, iDisplay lets you use your iPad, iPhone or iPod touch as interactive wireless display to give you more screen real estate.
Android version is cheaper - $1.47AUD:
https://play.google.com/store/apps/details?id=com.idisplay.v…