Spying is cheap: software and hardware cadets in a military academy learning to exploit specific systems. The main cost is wages, which are lower in China. For military tech, you need materials and engineering specialists. Often multimillion dollar prototypes just to find out where the problems with the techniques are. Specialized facilities for testing prototypes. De novo techniques for testing those facilities. Compare spywork, where it might be enough to bribe a single person in a chip fab to add a backdoor circuit.
Spying is not cheap, and the main cost is technology & science, not wages. Even in WWII the technology spies are definitely top tier geniuses with brilliant minds, not to mention now it's 2019 already.
