Nothing beats "research."
When I got out of college, I had a job which I supplied all kinds of garden products to several competitors of the "big retailers."
After a few weeks on the job, it started to amaze me how many times I saw competitors "checking prices" on my customers. I even got to know many of these "price checkers" by name.
So when we started our sign company, the first thing I did was "research," which basically meant calling or visiting "competitors" to find out what they were charging. We rarely "under-cut" our competitors, but really like to know what the market is charging. I've never bought a "price guide," simply because I've never felt these prices would truly (correctly or incorrectly) give me a gage as to what the market might bear.
We try to always charge prices that reflect our services, and it usually works. Almost always. "Live by price, die by price."
Good luck,
C.