Unfortunately, this “rule of thumb” of yours only applies when the task in question is on-the-order-of what your mother-in-law (i.e. a human being) deals with.

The fundamental fallacy of your argument rests in the fact that nearly all of the “hard” problems that a business-oriented computer must deal with are I/O-bound, not CPU-bound. Therefore, the justification that “CPU time is less expensive” becomes entirely specious since “the CPU” is barely even used.

The ruling constraint (to the hardware) is ... always is ... I/O and nothing else. Any strategy must necessarily focus upon the reasonably-efficient utilization of that resource, at least insofar as the entire computer-system is concerned.

The goal therefore must be to devote “only a reasonable amount of frighteningly-expensive human time” toward a strategy that efficiently avoids the hardware-constraint of I/O, even at the expense of (generally quite throw-away...) CPU resources.