"Semi-Guided Mortars inflict the same damage as Armor-Piercing (Shaped-Charge) Mortars" per TO:AUE p186.
Armor Piercing mortar ammunition increases the base cost of the carrying unit by 0.2 * ammo BV, per TO:AUE p198.
SG-M would increase the cost of the TAG unit(s) by the BV of the ammo carried. However, since SG-M works like SG-LRM and SG-LRM acts as normal LRM when no unit has been lit by a TAG, it appears one could simply take Semi-Guided Mortar ammunition without a TAG unit and avoid the upcharge for APSC. Is that correct/intended?