I think the company I started out in IT with charged around £15/user/month for the simplest sites but it did depend on the SLA, which I can't quite remember off the top of my head. It was definitely more for site visits and out of hours support.
It might be better for you to find a kind of banked hours type support model, just call them up when you need to and top up the hours when they run out. At least then it would give you a measure of how much IT support you needed and you could move to a recurring monthly support contract if you felt you needed it.