Economics is a real spoilsport for fantasy literature. I was once taught a single sentence which now nags at me in every book I read: "Describe how this army is fed." For example, the carnivorous Canim in the Furies of Calderon series fielded an army of some 60,000 apex predators in a sparsely populated agricultural region for a period of over two years. Their purchasing agent should require about as many sources of meat as all the McDonalds nationwide, but the economy as depicted couldn't possibly support that.