If you're not making that up it's just stupid thinking, probably relic overthinking things. "general reinforcement cost rule" LUL
Yes, that rule exists and it dates back to coh1 release over 10 years ago. It is one of the core rules that rule the game - its ment to promote team weapons over infantry spam to improve combined arms and to make sure bigger squads really benefit from being more durable as opposed to just being manpower sinks.
The rule is no longer in affect for infatry since the latest patch have changed the reinforcement cost, reinforcement time and build time of majority of unit units (that had their cost altered).
That is not exaclty true, only a few "elite" squads got their reinforcement ever changed.
The rule was not in affect even before when it come to HMG, since the reinforcement cost of maxim and Dshk was 15 while the unit costed 240 and 300 and the HMG-34 was one of most expensive to reinforce (even more than HMG-42)while being the cheaper.
There is at least 20 team weapons in the game. Only maxim, DShK and MG-34 don't follow the rule and in case of MG-34 it is just a 1mp discount (which still makes it the cheapest to reinforce mg in the game). MG-34 is actually cheaper to reinforce than MG-42.
Finally the idea of comparing cost of fully reinforcing squad is flawed, if one want to compared reinforcement cost vs small arm one has to compare by EHP (effective HP)of entity and vs explosive weapons HP of entity.
Comparing full reinfocement cost is the only solution. If you try to ballance the costs around single entity cost, then all you get is that bigger squads are disadvantaged by their size and become just a manpower sink. Remember that these models are actually weaker to compensate for the squad size. For example if conscripts were 30 to reinforce like grens, they would also have to be as good in combat as grens. Nobody would like that ballance wise.