The M2 Mortar and the RM-40 are in no way comparable to the Kz 8 cm GrW 42 or the BM-41. They have a smaller blast radius, SIGNIFICANTLY weaker damage output (can’t even pen 55mm or 30mm), and are outclassed in range.
But that is understandable since they are light mortars. So that leads to the question of why these two light mortars cost even close to as much as medium mortars. To answer that question, I don’t know. However, I do know that they should not be above a 100,000 xp cost.
If you want to add a medium mortar that costs 174,000 XP for the Americans, then add the M1 Mortar, which is an 81 mm mortar. But until then, both the M2 Mortar and RM-40 need to be lowered below 100,000 xp cost.