reduced cost of APC and M-SAM to $600

The GDI APC and the Nod Mobile SAM Launcher are the premier anti-air units that are needed to defend each side from air attacks. Due to the problems of orca spam, they should be given a more prominent role. 
Currently the APC is $700 and the M-SAM $750. This might be reasonable against a small amount of vehicles, but anti-air units need to be able to deal with spam. So they will be reduced in cost so they can be bought cheaper. This is important for the M-SAM especially since it's a dedicated anti-air unit.

To avoid the GDI APC being too good, the damage vs. heavy armor will be reduced a bit more, so it cannot serve the role of "light tank". It will still be able to fight light vehicles well, however.

mods/cnc/rules/vehicles.yaml

@@ -81,7 +81,7 @@ HARV:
 APC:
 	Inherits: ^Tank
 	Valued:
-		Cost: 700
+		Cost: 600
 	Tooltip:
 		Name: APC
 		Icon: apcicnh
@@ -475,7 +475,7 @@ MSAM:
 MLRS:
 	Inherits: ^Tank
 	Valued:
-		Cost: 750
+		Cost: 600
 	Tooltip:
 		Name: Mobile S.A.M.
 		Icon: mlrsicnh