Page reference please: where does it say this in the Mercenaries rulebook.
It says it in Hinterlands..
Page 16:
BattleField Support rules add air support, artillery
support, and other non-’Mech military hardware . The
BattleMech Manual and Alpha Strike: Commander’s Edition
contain the initial rules for BattleField Support, and the new
Mercenaries box set updates and expands these rules . If you
are using the older BattleMech Manual rules, double the BSP
costs listed in those rulebooks . If using the Mercenaries box
set updated costs, there is no modification needed .