Inside the glass, or on the shopfront?
These are different products. A display behind glass faces reflections and serious heat build-up in direct sun, which needs a brighter panel than the room would otherwise justify and real thought about ventilation. A display mounted on the shopfront outside is an outdoor installation with everything that implies — weather sealing, structure, permissions. Cheapest is often neither: a poster display just inside the door, where it's read by people who have already slowed down.