For residential IPs you can't even pay per month like normal VPNs, normally they charge per GB, usually over $2 usd per GB.
And I can get a semi-anonymous cable internet connection too (if your line is “hot”, you could sign up with any address… not sure if it has to be under the same node or just the same city). Would be difficult, but not impossible, to track down which residence the shadow connection is coming from.
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.