I can see no flaw in it. Yes, the Allies ARE ressource rich. Why the need for balancing that? What do you think, WW2 has been fought? Lebensraum?? Nice german cottages for all? Yeah...Oil fields in the Crimea, Coal in Poland and Donetzk, Rubber in Indonesia, Swedish Iron ore...you name it.
The only things you might not see is the following: the more democratic a land is, the more resources and factories are privately controlled, so the Allies need to pay for the products from tax money and war bonds. The Axis and Comintern (especially the latter) OWN both resources and factories or enact enough pressure on the factory owners to do their bidding. Also, historically spoken, those countries were known to use forced labour or similar mechanics. So no payment here, Mr. Doe.
The mechanism should henceforth take into account, that
a) The workers are paid. In democratic countries you order an item and pay with tax. Military productions are tax-neutral for simplification so you pay and get your hardware but do not earn anything in cash. Consumer goods (exported or not) serve a dual purpose. They make people happy and are taxed. This means, you need consumer goods. The need might be shortened by autocratic politics but you need to produce them for cash
b) Working people are taxed. The more people in a land, the higher is the tax revenue. Harsh tax laws might result in dissent.
c) Exports are taxed. If country A buys for example ore from country B the money does not go into the state coffers directly, but the mine owners get it and the state gets a share in taxes. In return, the mine-owners use the money to raise production capacity (reinvestment).
Internally the game could count the money earned for a special good and automatically upgrade the mines at a threshold value resetting the earned cash. The more totalitarian the government is, the more money directly flows into the state coffers, but this will make automatical upgrades unlikely. On the other hand a country could invest and subvent certain industries by paying tax money to a certain field of tech. This will make the AI build according upgrades.
e.g.: no chemical industry yet? Invest a certain sum of money and AI will build a chemical plant for you after some time.
Imagine a user interface where you can invest money into certain fields of resource production. The more totalitarian your system is the more freedom you have here as in deciding where the next factory of a certain kind will go. Drawback might be corruption making upgrades more expensive than in "free" countries. Appoint a minister to automatically appoint and allocate those funds (in democracies however, those blokes are elected). They might favor certain industries. Imagine, that Mr. Krupp would rather expand steel production than textiles...
You could also take unemployment into account. Unemployed people are unhappy (dissent) and cannot be taxed and maybe they cost money. They can however, be drafted for service (Military manpower bonus). Factories reduce unemployment but also drain the manpower bonus.
Stockpiling should cost money or, if stuff is not sold or used, make a reduction in production plausible (factories get closed because they are not lucrative or factory yards are clogged).