Ask HN: Forming an IT service organization, how much do I charge per month?

I was formally the IT Director for a chain of Retail stores (150+) - the company was sold and they eliminated my position.

What I have discovered is they have left their franchisees out to dry by not continuing their IT support. I was sure they were going to hire someone else, but they didn't. They are on their own. They have to find IT support locally, which from what I am hearing has proven difficult for them. The software vendor doesn't offer quality after hours support. There is definitely a need I can fill here.

About 50% of the stores are using Quickbooks POS and the other half is using a industy specific point of sale solution which I am an expert in.

What I would like to do is call every one of the store owners and offer my services, introducing my new IT organization. They know me, they all like me, and would be willing to pay a monthly fee I am sure of it. It easily a $100,000 a year business if I only charge $60 a month.

Where I am lost is:

How am I going to handle issues that require physical intervention? (Sourcing local contractors) Should I charge a call out rate for this and pay my contractor a 1/3 or a 1/2 and keep the rest? Better yet, how am I going to find these contractors and present such a job to them. If I go through another company, they are sure to charge me a high market rate for their technician.

Is $60/month enough for such a service? The phone lines would have to be available from 9am to 9pm Monday through Saturday and Sunday 11am to 7pm.

I need to keep my employees local, I cannot outsource. So if I hire a $35,000 a year help desk employee - I've got approximately $6K a month left in revenue to work with.

Common issues would be: My receipts won't print, I can't find report X, my database is corrupt, I have these stupid pop ups, etc. Easy stuff.

Thoughts?