$170 base price is a bit odd. Chromecast Ultra is $60, which means they are selling the controller for $110? For comparison, you can get an Xbox or PS controller for ~$60.
Ultra - $60 Controller - $60-70? 3 month Pro Subscription - $30 3 month Pro Sub for a Friend - $30 Destiny 2 Complete Edition - ~$100
So even without taking the game in account you're saving 30% of MSRP
Also, you don't get to own Destiny 2, just play it while you have the pro subscription (i.e. similar to Xbox Live Gold benefits).