Building academies does not ever reduce the cost of noblemen - they will have the same price no matter how many academies you control.
There is one situation something like what you have described, however, which is that when playing a world which uses the packages system to build noblemen (currently, this would be World 114 or the current High Performance round), you can rebuild noblemen killed attacking a village for just a single package. Therefore, on a package world, there is a situation where by rebuilding an academy that had been demolished, you could gain the ability to build a noble for just a single package. However, this would not actually reduce the cost of the nobleman - it always cost just a single package - you are just regaining the ability to build it.
