From: Iowan in MD/DC
PDU On actually costs more supplies and HI from switching the factories around to streamline R&D. With PDU Off, you can just let the default factories remain as they are and be largely fine, as you'll end up producing the airframes you need for your units but wouldn't use in a PDU On setting.
What would be a fair estimate of the supplies and HI saved? Would it be drop in the ocean or something quite significant?
Without divulging what I know of Obvert's production plans, I can give you some estimates, I suppose. In Scen 1, Japan starts with a number of R&D factories that people don't typically produce. I'll ignore factories like for the standard A6M lines, and D4Y/J2M/etc., as people are going to keep those factories in most cases. Likewise, the G4M R&D factories are likely to be changed to something else no matter what as getting the later models early doesn't make any difference.
There are about 19 factories for airframes that you'll need to fill out units. They're in R&D, but with PDU Off you might just leave them so that you can produce them later. PDU On really does let the Japanese player focus their aircraft production which at the same time allows them to focus their R&D - they won't later need those factories, or have to switch existing ones to those airframes that they changed the R&D factories for.
Let's assume the IJ player doesn't even go that far, and only has 15 factories that they wouldn't touch under PDU Off that they would under PDU On.
(Most of these factories are under size 5 to begin with, but some are larger, so would convert to a new airframe at a larger amount. I'd say average size would end up being 3. With optimal R&D size at 30, this means an expansion of 27 for each of these. The IJ player may expand some of these factories anyway under PDU Off, so let's say it's only 25.)
15 * 25 = 375. This is the amount of expansion for these factories, just once. If the player changes to research a different model that's not in the upgrade chain (say, because with PDU On they can just change all their Zero production to Sam production and use the R&D factories on the Karyu or something), this gets a lot more expensive. Not to mention the Production Factory expansion/repair costs from converting. Even worse, what if the IJ player decides that they want those R&D factories anyway and will just convert factories to the undesirable airframes as needed, later, when the squadrons show up? That's even more supply that needs to be burned.
375 * 1100 = 412,500 supply to repair them fully. This isn't a huge amount, but it's also non-trivial - about 1% to 1.5% of the total supply available to the Japanese in the whole war.
375 * 10 = 3,750. This is the initial HI cost to expand the factories. Trivial - less than a day's worth of production. Now, so far as I can tell, R&D factories don't burn up HI when they're researching.
Air groups will remain the same size no matter what, really, so we can call the HI cost of overall production to be a wash, for the most part. The exception to this is all of the 1E bomber units that many players will upgrade to 2E if PDU is on. This costs 75 PPs per unit, but it also doubles the HI cost of filling them out. A typical 27-plane sentai will cost 1,944 HI instead of 972 HI to fill out. Replacements will also cost double. Maybe there are 100 full Sentais throughout that game that need filling out and/or replacing. That's almost 100,000 HI more than PDU Off.
They aren't huge numbers, and this is a very conservative estimate - back of the envelope calculations, erring towards underestimating rather than over. I suspect the "Go bananas" impetus that PDU On gives the Japanese has a much larger effect on the longevity of their economy than I've just estimated. The HI cost of the R&D efforts isn't so bad, but the supply aspect is.