Is this be cause they're paying the residential proxy owners some of it?

Most of the people whose devices and connections are being used as residential proxy exit nodes are not aware of it.

They likely charge per GB because these residential connections are slow and limited compared to datacenter connections (doesn't help that they're often located in third world countries), and are often used for aggressive scraping, so charging a fixed monthly price would not be viable.

I can assure you they are not.