My 20,000mAh USB power bank which I bought for Switch and can also use for phones etc. was $19.99.
I would strongly recommend this setup instead of overpriced Nyko battery accessories.
Controller and dock seems OK though.
How much is it? The fact that it snaps onto the bottom so you don't have cables running to the power bank is pretty nice. I like how you can just leave it on, drop it into the dock and it'll charge both as well.
I'm cheap though so I'll take mildly annoying cables happily.