The cost of any false ceiling depends on two major items, 1. The material being used and 2. The complexity of the work needed.
The material cost is directly corelated with the area of the false ceiling, so will always be indicated as a multiple of the area. The complexity is more related to the length of the edges. Hence for any false ceiling project there is a per sft cost and a per running feet cost. The first depends on the material used and the second on the complexity of the craftsmanship needed.