It's completely free to use, no ads or in app purchases. I didn't like what was out there and so I made one better. It allows you to set custom percentage price alerts, like if bitcoin moves 5%, and it's price is $50K, it will alert you when the price goes to $52.5k.

It will then use 52.5K as the new basis for a 5% alert.

Currently supports bitcoin and ethereum. If there's demand, would add other cryptos.