This depends on the different standards, the price of transformers with different safety standards varies a lot, because the insulation requirements and effectiveness are so different. Isolation transformers for medical equipment are the most expensive because of the 4KV withstand voltage requirement and the leakage current requirement of less than 100uA.
The price of an ordinary isolation transformer 1 dollar 1VA, medical may reach 1.5 per 1VA, wind energy high-voltage control system transformers reach 1.8 per 1VA