PowerDown charges a flat 25% of what you save on your cloud bill by powering down resources. If you do not save, you do not pay.
PowerDown counts the compute portion of your cloud bill for each resource — network and storage components are not included. When a resource is turned off by PowerDown, the savings are counted until the resource is next powered up. Of those savings, 25% is charged by PowerDown and will be invoiced at the end of the month.
After 7 days, PowerDown will stop counting the resource in its savings calculations. Thereafter, you will not incur any further charge. So you can safely use PowerDown to turn off any resource for a long duration. Note that AWS will terminate RDS databases that are powered down for longer than 7 days.
For definitive information about pricing, please go to PowerDown Pricing.