Residential proxy bandwidth is extremely expensive, comparatively speaking. It can be up to $1 per GB but is more typically about $0.20 per GB.

Most users of residential proxies just get a SOCKS5 address and routing, they don't actually get computational resources of the infected systems beyond that. The user of the proxies, the operator of what the article describes as a control node, would be the device responsible for the PoW.

Do you have any evidence that AI providers aren't using residential proxies?

If you pop my machine and use it to route 100 MBit/s, I might not notice for months.

If I hear the fan spinning at night, you're probably getting caught immediately.

If you pop my mom's TV box and use it to route data within the connection's capabilities, you're getting away with it. If you consume a little bit of resources, still. If you consume enough to be useful for these kind of challenges, chances are her TV playback will start to stutter, which will be resolved by taking the compromised TV box, and removing the malware using advanced mechanical means called "a trash compactor".