Update [12/7]: eBay with Google Store has it for $15 - 10% off with code PHLDAYTEN [Exp 12/8 at 3AM PT] = $13.50 with free shipping.

Best Buy has the Google Chromecast Audio, 2015 model (Black, RUX-J42) on sale for $15 with free shipping or in-store pickup. Google Store and B&H Photo Video also have it for $15 with free shipping.

New customers on Google Express (Best Buy) can get an additional 20% off with code HOLIDAY18 [Exp 12/31].

We really liked the Chromecast Audio (9/10) on The CheckOut, but it's more for users of Android than iOS.