I am not looking for a solution right now but if I did, Yes I think a small charge of few bucks a month may work. The problem is that most small blogs don't really need heavy commenting and when they do, they can get away with free comments from WordPress or even disqus.

You need to think about the value proposition of why someone wants to PAY for a commenting system. It may not always be "I hate ads". I would say find people who run blogs with comments and start talking to them.