Existing production can matter more than a weapon's paper simplicity

Are simple multiple-launch rockets cheaper to build than ordinary 152 or 155 mm high-explosive shells?

Not necessarily; their immediate advantage may be that production facilities already exist rather than that each rocket is inherently cheaper.

The question compares simple multiple-launch rockets with conventional 152 or 155 mm high-explosive shells. A rocket is not automatically cheaper merely because its design appears simpler. Its practical advantage may be that facilities for making it already exist and are ready to deliver rounds. Wartime cost and availability depend on installed production capacity as much as the theoretical complexity of an individual weapon.