If you look at the list prices for the intel lines and then Apple's corresponding price increase to upgrade the CPU, it's readily apparent that the cost of CPUs are not the true reason for the prices that Apple charges. Apple would only be considering switching to AMD for a technical advantage, they have no concerns with passing on their costs on to their consumers.
OK, but if you look at the evolution of Apple machines, you'll see that Apple has replaced CPUs with newer ones which were similar priced and which had a similar TDP. Apple seeks to maintain its profit margins. If Apple changes to AMD, it will be to save its own costs or to increase performance for the same cost. Macs will probably not get cheaper.