The sender doesn't have to pay a lot to make it effective. Remember that the spammer is sending email to a hundred thousand addresses per day, for free. If you charged even $0.01 per email, that'd stop a lot of spam right in its tracks.
However, some companies sending spam^wnewsletters would be willing to pay. I think this crap needs to stop, too, so this would be a partial solution at best.
I don't think it's necessarily about the amount you charge but that for charging it you need access to a bank account / credit card or other banking infrastructure. That's not the case for everyone taking part in the internet.
Or proof-of-work, so long as it can be done without invoking The Blockchain (proof of work can be done without a blockchain, but you need to invest in armed guards to shoot anyone who mentions it).
However, some companies sending spam^wnewsletters would be willing to pay. I think this crap needs to stop, too, so this would be a partial solution at best.