Every habit has a rate, and you can change it.
Open the habit from its quest screen and you'll see the rate under the name. Focus habits show an hourly rate, quick habits show a flat payout. Tap through to change it.
How to set them sensibly. The rate is a price you're setting for your own time, so ask what an hour of that habit is worth in scrolling. If reading is worth an hour of Instagram to you, price it accordingly.
The trap. Setting everything high feels generous on day one and hollows the whole thing out by day three, because coins stop meaning anything. If you find yourself never running out, your rates are too high.