Percent pricing for configurable options/add-on packages would look something like this:

Lets say a package costs $10, and there is a configurable option for extra backups that costs 20% of the package price, making the order $12 total.
Another package costs $5, it has the same configurable option and when selected, it costs 20% of the package price, making the order $6 total.

At the moment, you can do this in a hack-y way, you have to create a configurable option for each package you want to put this on, and manually calculate its price, instead of reusing the same configurable option.

Taking inspiration from cloud providers, they use this model for their backups, enabling backups makes you pay a fixed percentage more on top of your current VM price.
Hetzner Cloud for example:
https://imgur.com/hXnGeWp