I know I'm not from the United States and things are generally handled differently when it comes to relationships with GC's but what your talking about happens up here alot.
It has gotten worse over the years and I will not play that game, maybe to my demise (20 years this year in buis. and seriously thinking about closing it down, but I have 15 employees to worry about).

Anyhow it became an obsession with me to find ways to stop it (no luck so far)but i was surprised at the documentation and studies in the United States related to "Bid-Shopping", I did a Google search for bid-shopping and was really surprised.

It is against the law and best purchasing practices, in Canada and I'm pretty sure in the US, problem is knowbody does anything about it.

Here is how I see it:
It cost's money to generate a quote.
The quote is issued at the estimators best "guess" as to what the project is going to cost.

Then along comes some "BLEEP" and steals the job by saying or thinking "Oh, yea. I can do that job for that price. I know ABC Electric issues good prices for jobs." Of course he's ahead he didn't have any overhead cost's related to pricing the job.

Sorry, got carried away, it just irk's me to no end when this happens.

I have thought of putting up a WEB site where good contractors could post the People who practice "bid-Shopping or un-ethical pratices" but have advised by my Lawyer that it would probably be very hard to prove that it was happening and "SLANDER CHARGES WOULD REALLY HURT".

For me, I have Morals and believe "WHAT GOES AROUND COMES AROUND". I can sleep at nights, maybe on an empty stomach but without guilt.