I have been having work done on my primary residence and have found that multiple offers will give you a wide spectrum of prices. One discovery from Biggerpockets that has helped me is the two books by J scott, sold here on Biggerpockets. The first one is "The Book on Flipping Houses." It will aide in determining a strategy and set up a system you understand. The second book, "The Book on Estimating Rehab Costs" is the one you need now - It expands on the first book but it breaks down the specific jobs and gives you a breakdown of individual components.
I was able to get a better quote on repiping the supply lines to my house because of what I learned from this book - particularly the understanding of what the contractor, or in this case plummer, uses to write a quote - In my case I used what I learned about cost estimates by knowing the number of fixtures and I was able to get a quote from a local plummer about 2.5K$ lower than the highest quote. It helped to talk to the estimator about my expectations and listen to their spiel and ask questions specific to what your stated expectations are. So far, the quote is in and holds for 30 days and work will be scheduled when I have my master bed remodel started in mid August.
You can find these books in the Resourses section or, what I did, was purchase from Amazon.com.
