I'm not sure what's going on here.
ONYX App's Spectrum Analyzer is usually $15. It was on sale last week for $10. The price came back up to $15 on Friday and now the price is down to $0.99. The app has never been that cheap before!
I wonder if this is an incident of price bouncing; a practice used to artificially keep an app relevant by frequently changing the price. Perhaps something went awry and they missed a digit when trying to set it to $9.99!
Here is a demo video from the developer.