99$ for diagnostic from a dealership is not highway robbery. Personally, I would have to trust an independent shop ALOT to work on my car. IDEALY, the shop rate is paying for trained technicians to perform the highest quality work on your vehicle. I give my customers the best service I can offer because I prefer job security than ripping customers off.

Anyway, I would check the linkage to the inner door handle. If the little plastic retainer near the end of the cable is broken, it won't stay locked into the holder and, though the handle is hooked to the cable, it wont open.

I have replaced countless door lock actuators in my time. It is likely failed. You can unplug and apply 12v to the door lock actuator, ( not the switch ) for just a moment and see if it works. This is a good way to test the actuator.