as the politician would say.. " I prefer to look at it as a bombing tax"
actually I don't think bombs / normally carried ordinance would be perked... just some of the odd munitions / guns.
Say you want the 75mm gun on a ju87... slight perk to add that gun,
maybe a 110 with the cannons that fired directly upward - perked...
or you want to up a C-hog...slight perk on the D version giving it the cannon load-out.
maybe you have a flakpanzer for free, but you can get the wirblewind for a slight perks
I would think it would be based on situations like that.