Most bots and scrapers from what I’ve seen already are using (headless) full browsers

That's not going to be the majority of your bot traffic by a long shot because it doesn't scale like using basic HTTP requests.

This is from personal experience. With PoW you just need any puppetted browser, maybe less. With Canvas finerprinting you need a heavily customized scraping browser, either one you made yourself or one you're paying for. If that's the case the cost of PoW is neglible. If you still want actual stats, I'd have to ask where you're getting any stats on PoW working.