I wouldn't settle for those figures. $75 a month is roughly equivalent to two hours' work, and that's on the shy side. (I know plenty of design/dev folks who'd charge that or more for a single hour.) You go in with that quote, considering that you're managing the site, and you'll find yourself working for field hand wages. And $600 is a big lowball figure (at least in my estimation).
Remember, the site is for a business, and their ultimate business goal is to maximize profits. That includes paying you as little as they possibly can while they squeeze the maximum amount of work out of you. Your goal is to make as much money as you reasonably can for the work you provide, not to give these people what is essentially "free" upkeep.
I do two kinds of jobs: paid and pro bono. Pro bono gets done without a mention of money, but it takes place on my schedule and according to what and how much I want to do -- it's like working for Habitat for Humanity, if you don't want to work the jackhammer, you don't. Paid jobs are clear in what I will do, what I won't do, how much I'll get paid for it, when I'll get paid for it, all the way down the line. The potential for abuse by your client if he blurs the lines between a client relationship and a friend relationship is enormous ("Gee, why won't you redo the entire navigation scheme tonight as part of your normal maintenance fee? I thought we were friends."). You did the right thing in coming in and asking about this, because you could be heading for a financial bruising.
Edit: Something that might interest you.