This is not always true, but I believe it's the default rule, and it often holds true. For example, Conscripts cost 20MP to reinforce because 240MP / 6 / 2 = 20MP. Likewise, Grenadiers cost 30MP to reinforce because 240MP / 4 / 2 = 30MP.
Then we get to weapons teams. If we follow the rule above, Maxim teams should cost 20MP to reinforce. However, they actually cost just 15MP to reinforce. Likewise, the Soviet mortar should cost 20MP to reinforce, but it also costs just 15MP.
Before I get accused of Axis bias, I checked the Ostheer weapon teams as well. The MG42, for example, should cost 32.5MP to reinforce (260MP / 4 / 2), but it actually costs a bargain 22MP to reinforce. The same thing goes for the Ostheer mortar, which should cost 30MP to reinforce but actually costs just 22MP.
So my balance solution is pretty simple: adjust squad reinforcement costs - for all factions - so that the rule listed above (squad cost / number of squad members / 2) holds true in most, if not all, circumstances. Obviously this wouldn't represent a monumental change, but I think it would be a move for the better and would push support weapons teams back into their intended support role.
Actually the formula does hold true, you're just missing a key piece of it: the weapon itself. The crew of a Maxim Machine Gun costs 180 Manpower, and the Maxim itself costs 60 Manpower, for a total of 240 Manpower. That also means that 180 / 6 / 2 = 15. Same goes for the Mortar which is also 60MP, or the ZiS-3 Anti-Tank Gun which is 140MP.
The truth is simply that Weapon Crews are just cheap, their rifles or submachine guns are near-useless, so they exist to use a weapon and they are relatively disposable. They have 25% received accuracy which means they die much faster than regular Infantry, so they need to be cheaply replaced.
Maxim spam is silly and unfun, but the formula you mentioned is definitely in use. Maxim Spam is mostly the result of Conscripts being the worst scaling core infantry in the game. Grenadiers get MG 42's, Volksgrenadiers get Panzerschrecks, Riflemen get 4 weapon options, Conscripts get nothing.