Interesting stats, but don’t think it weighs too heavily on what units one should purchase. Each piece has it’s particular pros and cons, and generally the higher cost (IPC to attack/defense strength) units have a special capability not available in a lower cost unit.
In a perfect world, I would like to go into a ground battle with a mix of inf, art, tanks, and air, because they each bring something special to the battle. Likewise, I think naval battles are best conducted with a mixed fleet. The reality, however, is that other considerations tend to skew the optimum attack or defense force. Many times, you simply have to fight with what you have on hand.
That being said, buying the cheapest unit that will do the job is not bad advice, but quite often the special capabilities of a unit matter as much, if not more, than the cost.