Depending on many factors like :
Cost of raw material
Sale price
Technology used
Economy of scale
If a power plant with free ash sets a plant with new technology that uses upto 95% ash as raw material and goes for economy of scale of say 1 to 2 lac bricks per day. The cost of production will mainly be the cost of finance of plant. Typically the cost of brick will be less tha. Rs2 and that of 6 inch block about Rs 5 . The payback can be as low as 8 months to a year at most.