Amazon is really pushing its Android Appstore now that the Fire TV is a thing. Even if you don't have a Fire TV, you can get plenty of free Amazon Coins from downloading free apps, plus extra Coins when you buy things. The Plex app is usually $4.99 in Google Play and Amazon, but it's down to $0.99 (or 99 Coins) right now.
Plex is a media streamer app, but that's only part of what it does. The Plex app connects to a server installed on your PC that indexes and organizes your media library. It can even transcodes your media on the fly so it plays on whatever device you like. It works with Android devices, PCs, and the Chromecast. The app gives you access to the basic features, but a Plex subscription is required to get premium and early access features. That includes things like cloud streaming and photo backup.
The Amazon deal will only drain your account of 69 total coins because of the 30 free ones you get for buying. If you take advantage of recent deals, that's essentially free.