There is an offer currently listed for 75% off a movie rental. When redemed it also mentions TV rentals too. Showing in my account as "75% off any one movie or TV rental".
To check for it, find the Offers section on Chromecast App (or Google Home app) or at the Chromecast Offers URL (click Get Deal or go to http://www.chromecast.com/offers
Available until Dec 31st 2016 but can be used until the end of Jan 2017