Take your average hourly wage, and cut it in half, because that's about what an untrained installer is worth on any job. Even if you know what your doing very well, you can never be nearly as good as a well equipped install vehicle which should amount to a shop on wheels. Also, while you fail to account for overhead etc. If you do work on the side it's less than what a shop would charge, so you can't draw a parallel their either. We constantly take on the work of cleaning up installations and hook up of people that otherwise have good intentions, but can't maintain their value, not meet their full expected obligation.

Not trying to sound bigger and better, but while I'm a very proficient craftsman in many trades, I would never try and draw a parallel between my self and the rates a carpenter would charge for instance.

As always, if you think it's expensive to hire a professional. Just wait and see what it costs to hire an amateur. And certainly I'm not trying to apply this thinking to a 2-4 hour hook up as you are likely describing, but often times people under estimate the complexity of a professional install. They can regularly fall in the 10-30 hour range for a decent surround system trim out.

If you want to do work for a friend from work, do it as a favor with him buying the food and beer, and don't pact it in right away. If he's not a friend, do it for a fixed amount that he can agree on ahead of time. One decent rule some trades people must often stick to avoid getting taken advantage of is "I can't do work for friends and family, but I can recommend somebody excellent to you." And no. You must still do work for Mom and Dad.