Fair question! Pricing for compute is hourly, so if you stop the paid instance you're no longer accumulating a charge for compute. The storage costs are monthly and based on the size of the disk.
You stop paying for the disk when you delete an instance, and you will only be charged the fractional amount based on the number of hours that instance existed that month.
You can also see the charges you've accrued so far in the top-right menu.
Let me know if that's clear 🙂