Let’s assume that 1 IPC equals 1 million man hours of production. I have some data on production per man hour that I can plug into that.
In the US, for aircraft, 1 man hour of production produced 1 pound of airframe weight or 1 horsepower of engine. A P-51 was then roughly worth 7500 man hours of production, a P-47 worth about 14,000 man hours of production. Call it that a standard fighter would cost you 10,000 man hours of production. Fighters cost 12 IPC in A&A Classic. 1 IPC buys your 100 fighters, so 12 IPC would be you 1200 fighters. A B-17 would cost you about 40,000 man hours, so 1 IPC buys you 25 B-17. Fifteen IPC buys you 375 B-17, or about 10 bomber groups.
A Sherman tank took about 2,000 manhours to assemble, or one man year. One IPC buys you 500 tanks, 5 IPCs buys you 2500 tanks.
A battleship would take 3 years to build, using about 2,000 men, or 6,000 man years. A man year is 2,000 man hours, so a battleship should represent 12 Million Man Hours, or 12 IPC. In the game, battleships cost 24 IPC, so each ship miniature represents two battleshps. Main problem with battleships was armor and turrets, so building in less than 3 years is tough, unless you do a lot of prework.
A carrier could be built in 2 years, or 8 million manhours, or 8 IPC. In the game, 18 IPC gets you two carriers plus change, so adding the air group to each carrier is reasonable. Figure 10 destroyers equal one carrier, so for 12 IPC you get 15 destroyers. So far, not too bad, the numbers are reasonable.
Next is the transport. Using mass-production methods, you could estimate that one man could produce 100 Gross Register Tons, a measure of cargo capacity, in a year. A Liberty ship was 10,000 Gross Register Tons capacity, so 100 man years or 200,000 man hours. For 8 IPC you get 40 transports, 5 per IPC.
I do not have good information on Infantry costs, but that would be considerably different for the US verses everyone else, as the US essentially motorized all of its divisions, unlike the Germans, Russians, and Japanese, all of whom depended heavily upon horse-drawn transport.
Now, these production rates are for the US. British production rates were a tad lower, as were the German, with the Japanese rates a lot lower, less than half of the US rate. I have no adequate information on Russian production rates.