[Imp]

Just to clarify the ROF thing.
Lets assume arty is mainly used plotted & not Z firing which you might do with Light Mtrs perhaps in some cases SPMTRS or in desperation in self defence.

The mortar will then have a higher rate of fire at small call times so for all intents & purposes the ROF is about the same.

On cost you could flip the argument & say the mortars ability to be directed & fire smoke are quite cheap. Balanced by lower speed & ammo which if you wish can be compensated for by buying a truck or some ammo resupply.
Of course this makes arty effectivly expensive as it needs a supply train & observer to operate effectivly over time.