Searched and found no one had posted this deal, if they have, sorry. Groupon deal for the original 3DS Power Plus battery, regular price $20. ($9 on Amazon, but they only have 4 left) Not sure how fast Groupon works since I've seen people mentioning they take their time.
Here's a comparison piece between this and the Nyko+, which requires installation, while the Hyperkin does not. Mixed reviews, but someone might be interested at this price.