Depending on your financial background $1 could already be quite a bit. I'd prefer to charge just "not nothing" which should be below 50c for me. Preferably below the costs of a text (costs ~9c here).

The bigger problem however is what ever the procedure for signig up is. Even if it's just paypal and a webform for the mail.

I guess the problem you'd see with that is exactly how this service is differentiated. Yes, it blocks spam ( but so do many other things ), but that's more of a side effect.

It's really about purchasing access to busy people. As you say.

That's an understanding I have anyway. Please correct it if it's missing something.