All of the computation, including pre-processing, is offloaded to the TPU. The weak machine is really just idling. A bigger one will only cost money and have no measurable effect on the performance.
What is the cost difference between the CPUs on the google cloud vs AWS? How would adjusting for it effect the cost/images ratio?
This is why my previous comment mentioned that GCP is a better benchmark for this since you can select the number of CPUs to match with the GPUs to some extent. You can get a rough idea of the savings by looking at their P100 instances.