Here is my thinking why it should be limited to 1 cheap unit per turn:
There’s no problem with having unlimited cheap German subs, British fighters, Japanese transports, and US bombers. The reason why is because with all these unit types the logic doesn’t hold that if it’s beneficial to buy 1 cheap unit, then it’s beneficial to spend all/most your money on that cheap unit. Germany can’t buy all subs and win, US can’t buy all bombers and win, etc… What I’m worried about is unlimited cheap Russian armor. If it’s baneful to buy 1 armor because it’s cheap enough, then Russia should probably end up spending all their money on armor.
Obviously, you can’t have unlimited armor at 3 IPCs each. 1 armor per turn at 3 IPCs should be fine because still many infantry will be purchased. I don’t see how 1 armor per turn at 3 IPCs would really make Russia that much stronger, but we’ll never know for sure until playtesting.
The only other option for cheap armor is then armor for 4 IPCs each, either with some per turn limit or not. If we have a per turn limit where armor can be 4 IPCs each, then the worth of that NA would only be approximately 1IPC*(the limit of cheap armor per turn). In the case of 1 per turn, the NA would only be worth at most 1 IPC per turn… not very good compared to the rest of the NAs. This then brings me to why I don’t think unlimited armor at 4 IPCs each would work… If armor costs the same as rtl, then the mass production of armor would cut into the production of rtl. How much rtl would you buy as Russia if you could pay the same for armor? I for one wouldn’t buy a whole lot of rtl, if any. If you take into account the infantry attack bonus for rtl, then they are at best just as good as armor. On defense and movement, armor is clearly better. Realistically, there must be some benefit for Russia to purchase rtl and there isn’t a benefit when all armor costs 4.
So I guess my conclusion is that the best way to model cheap is to have a limit of 1 per turn at 3 IPCs. Alternatively, you could say, if Russia buys 3 armor per turn, then they get 1 free, but the problem with that is that it doesn’t compliment the other cheap unit NAs very well. You can’t say, for every 3 fighters UK buys in a turn, UK gets 1 fighter free. UK would have to save up a lot of IPCs and then buy 4 fighters in 1 turn. That’s not a good realistic purchasing strategy.
I feel that a limit of 1 per turn brings a nice similarity and simplicity to the cheap unit NAs, while offering a great compromise between realistic mass production and game balance. I concede that it doesn’t make perfect sense to have a limit on production like that, but think of the effect of only having 1 cheap armor per turn on the game. It is a great, simple rule that happens to give a realistic model of the number of Russian armor units without being too powerful for Russia.