How do you know if a contractor is ripping you off?

We are now deep into the land of contractors, and I am lost without a map -- how do I know if I am getting ripped off? We are getting about 25 new or replaced outlets/switches/fixtures, and I have an electrician estimate that comes out around $95 per. How do I know if that is a fair deal or a total ripoff? Any thoughts?

More generally, here is where we are in contractorland:

1. Chimney people come on Thursday to close up the gratuitous hole in the kitchen, clean the flues, and seal up the firebox so we don't set the house on fire. The good news: we don't have to replace the sagging brick hearth. Quoth the chimney guy: "I've seen worse."
   
2. Plumbers kick off next Tuesday by replacing our entire stack from 3rd floor to basement. Right now it's 4" iron which sticks out the wall in ridiculous ways; we are having it replaced with 3" PVC that should stay in the wall where it belongs. From there he will move to kitchen and the other stuff in the house.
   
3. Tree guy gave us a first estimate. We are going to:
   
   
   
   • Prune back the front lawn magnolia so it doesn't swamp the house;
     
   • Cut down the front lawn cherry tree, which is a) ugly and b) in its declining years; and
     
   • Raise the gigantic back yard maple by around 20' by cutting off the large low-hanging branches. This will make a huge difference, since as-is today, the lawn is barely visible from the house because of those branches. Our neighbor hates this tree -- apparently it doesn't get leaves until May, and doesn't drop them until December.
     
   
   
4. Electrician will hopefully be lined up once I figure out the fair-price problem, listed above.
   
5. Carpenter to set up the fake wall -- still need one, hopefully will make some calls in the next couple days and get that going. I am kind of counting on the fact that building one stud wall should be trivially easy work for a decent carpenter.