Probably no answers as no one has access to your camera to fiddle around with it. Easiest solution would be to send to Nikon repair center. My guess would be broken aperture lever on the body. Unless you've got old primes that have to be locked into the smallest aperture; vaguely remember that causing troubles back in the day, but I haven't had one of those lenses in years now so that's just a shot in the dark. Cost-wise, again...no one can really give an accurate estimate short of those that work for Nikon or other authorized repair center. In my experience no camera repairs that involve going into the body and removing/replacing things are cheap.