You pay more because you reinforce less.
If we assume that everyone reinforces from 1 man left in the squad, (And all units follow the traditional reinforcement formula because I haven't memorized this shit.)
Grenadiers reinforce 3 times for 30 mp each = 90 manpower
Rifles reinforce 4 men for 28 mp each = 112 mp
Tommies reinforce 4 men for 28 mp each = 112 mp (They are marginally more efficient if they don't get 5 man thanks to the cost reduction forever ago)
Conscripts reinforce 5 men for 20 mp each = 100 mp.
Volksgrenadiers reinforce 4 men for 25 mp each = 100 mp.
You also have to consider how much each faction is reinforcing. Ostheer usually is support weapon heavy, which don't need to be reinforced as often compared to infantry. And when you are choosing a build order you need to consider how badly manpower bleed will hurt you. Going for a 4x Grenadier build? Then you better be really careful to avoid big losses. Additionally, the nature of LMG's means most of your DPS is concentrated on one model meaning that losing models isn't nearly as detrimental to combat effectiveness as it is for other units.
All weapon upgrades are pretty much no brainers. Except for AT ones of course. It's a matter of prioritizing your munitions or choosing between multiple options. (I.e paratroopers picking between thompsons or lmg's) However unlike say Riflemen or Tommies, Ostheer doesn't have to tech consciously towards their weapon upgrades and pick them up manually. This is why you see a lot of players neglect weapon upgrades as USF or UKF, because they forget to buy them.
That's the worst excuse ever!
Grens need less to fully reinforce only when they can die less frequently than other Core infantry
In fact no, Grens aren't more versatile than Volks, Rifles or Tommies..
They will suffer casualty in first salvo of changing fire