I haven't been the biggest fan of this game so far (I got it through Kickstarter), but $15-20 is a totally solid price for this if you're a fan of the old Banjo games.
The game plays really well (they definitely nailed the movement and platforming controls), but the levels haven't been as good as BK1 for me (the first level has actually been the best out of the 3-4 I've played so far).
It's a one day special at Best Buy, it'll go back up on Monday ... not saying a permanent drop won't happen soon, but they do this periodically with games. It's still $39.99 on Amazon, though I expect them to price match at some point.
I went ahead and ordered it at that price ... why not?
This didn't get very good reviews or word of mouth from what I remember so that probably didn't help. I certainly haven't moved on, I'm just waiting for the Switch version and I think a great game is just a great game to people so if one of these types of games came out that got amazing reviews/word of mouth it would have did better I think.