My experience is that a shop will charge 1 hour to look into this problem. These types of problems can be very difficult to verify and diagnose however, which means in some situations it will take more time.
Most shops will charge you an hour's labor to do diagnosis. In the case of electrical problems, sometimes diagnosis cannot be completed in a single hour, and the shop will tell you that.
It's smart to put a cap on it. Diagnosis should really be possible within 2 hours. You might try negotiating with the shop . . . if you get the car fixed with them, ask them to apply the diagnostic hour toward the repair they recommend.
You can do this yourself. By pulling all fuses and having a tester. Make sure battery is fully charged. turn your tester to voltage test all fuseable links, look to see which will be on a constant power source like dome light, or head lights. you wont receive any power from those points until switch is on. if you get a continuous power from ac, or another link thats not supposed to have power thats your problem. most cases of this is faulty wireing, radios, amps and bad grounds. In all other cases start putting fuses back one at a time till you notice voltage drop from battery. Trust me this is the easiest way.