Hi Rod1.
I think the main point here is do you keep the aircraft outside or even in a leaky hangar because avionics don't last long in the UK due to our wet climate. Not so long ago I had a King KY97 fitted to my aeroplane and was appalled to see that during the following winter, even though the aircraft was hangared, condensation would form on the inside of the digital display window! You can guess how long the unit stayed serviceable after that! Now, don't ask me how you'd do it, but I think the avionics manufacturers could really do with looking very closely how they could make their products better 'proofed' against damp and condensation. I suspect that, because Narco and King/Bendix are US manufacturers they don't really get alot of problems in this direction. As to how much your repair cost. It's been my experience that licenced avionics engineers are as rare as Hen's teeth in the U.K. and they can charge pretty much what they feel the market will bear. If you find one that does a good job for a reasonable charge then stick with them.
Spiney.