Yeah, 501 Resource Hydras would be more prohibitive than 500 Gold + 1 Resource.
I'm game, this sounds interesting. The one tweak that I might recommend, is to consider removing "capital only" restrictions. Since this affects mostly sacreds, and most sacreds have high Gold cost, a Gold+Resource cost would make it impossible to recruit very many from any one castle. Of course, commanders could be left as they are, and not affect the gameplay.