It pays to shop around. You'll see numerous listings for garage door repair companies in the phone book...although the names/phone numbers are different, they are the same company. Notice that a lot of the companies do not list a physical address.
Be careful about signing any open-ended orders. One trick seems to be
surprising you with a bill for replacing a lot of parts (rollers, drums,
etc.) that you don't really need, at extremely marked-up prices
Ask your friends and neighbors who they've used...and would they recommend the company.
The total approximate cost can run from $100 to $200, depending on whether you have one or two springs replaced.
Be certain you call around for price comparisons before hiring a contractor. Getting a referral from your friend, neighbor, or even your trusted real estate agent is a great way to go. Some unscrupulous companies may charge many hundreds of dollars for this job, which takes about an hour for them to complete.
NOTE: Check with your local Better Business Bureau to be certain your garage door repair company doesn't have any claims. It's also a bad sign when you call ABC Garage Door company to fix your garage door spring and a guy driving an XYZ van shows up, or worse, an unmarked van shows up. These guys often charge extra because they are earning a commission from ABC on the work they do.
http://ezinearticles.com/?Garage-Door-Repairs-and-Selecting-The-Right-Company-For-The-Job&id=5374477
Great article on how to do it yourself:
http://www.truetex.com/garage.htm