I don't have one, I feel I don't need one, and no one has ever complained about me taking too much to reply to something. As a matter of fact, as I take my time to reply to something, I usually change my mind about what to say, about what's important and what's irrelevant...

And, no, $100 a month isn't nearly close to an acceptable price to mitigate random "complaints" about a possible "reply lag"