Microphone and speaker replacement should not cost you that much money compare to other services available for macbooks. It is recommended that when replacing the microphone and speakers, also get a Professional Cleaning on your device. Some shops charge you half price for this service when you are replacing the microphone and speakers because they have to do half of the job regardless.
Sometimes the Problem is not just the Microphone or Speakers. There are many chips and chipsets involve in the audio and data transferring process. Logic-Board repair is then needed if that is the case.