How much to charge for this service?

If this was me in my market (all markets are a bit different), I would charge a set up fee and an ongoing maintenance fee.

I would not set this up for less than 100 bucks, and then only if I am using it as a gateway to other services. If this is all I am doing for them, I'd charge between 200 and 500 (small practice versus large practice).

For ongoing maintenance, I would make them set up their own autoresponder account (I mean I'd do it for them, but it is on their credit card and in their name). I would run that through my affiliate code for the autoresponder company and earn that monthly commission. There are several ways to bill for this - flat fee per month or per updated message or per contact fee monthly or per message. What's your time worth. Bare minimum I'd charge is 50 bucks an hour (and if it just involves posting messages, I'd outsource that at 8 - 10 dollars an hour). This should be earning you 50 - 100 bucks a month bare minimum, much more if there are frequent messages.

With that said, keep in mind that all markets do not handle that (and some will handle much more). You have to take your market into consideration, so please only use the above as a general guide, nothing more.

Also, this could be a good gateway to an SEO package or some other marketing package. SEO for chiropractors can earn you anywhere from 500 - 2000 a month (depending on the size of the practice).