UPDATE: Before going to the pick 'n pull or other junkyard, price out the parts you are looking for both online and at local stores. Some junkyards charge more for used parts than retailers charge for new or rebuilt parts. Some new parts can be remarkably cheap!
Second, don't let them pull the old "bait and switch" on you. I've read countless horror stories about yards that will quote you one price for a part, and when you go pull the part, they then say a different price. Some people, after spending an hour pulling a part from a car, give up and pay, instead of walking away - and end up paying more for a used part than a new one.
Also, beware of the multiple parts game - they quote you a price for a part, and then claim it actually comprises several parts, and their price was only for one portion. So you get an alternator, and they claim the pulley and mounting bracket are separate parts for an extra charge.
Don't be afraid to leave the parts on the counter and walk away. Odds are they will cave in, and if not, well, you are better off buying new or rebuilt parts and letting them go out of business.
Used parts aren't worth much. The real value in a junked car is the scrap metal and any money they make selling parts off a car is just found money to them.