One of the space-saving and alternative options for wastewater treatment is the use of Membrane Bio-Reactor. This paper delineates the sequential approach to explore integrated cost functions for estimation of the forecast level cost for biological treatment of wastewater by use of Membrane Bio-Reactor. The cost functions for capacity wise small range, medium range and large range groups of wastewater treatment plants with Membrane Bio-Reactor have been developed based on process design and cost estimation. It has been established through regression analysis that they are best expressed by polynomial equations. Validation of cost functions has been executed by assessment of mean absolute percentage errors. The respective determination coefficients in respect of cost functions for capacity wise small range, medium range and large range are 1, 0.9995 and 0.9996. The respective mean absolute percentage errors with reference to the cost functions for capacity wise three different ranges are 0.18 %, 0.82 % and 0.63 %. In each case the value of mean absolute percentage error is well below 10 %. This study has established that accurate forecast level cost may be arrived by integrated cost functions developed for capacity wise three different groups of wastewater treatment plants with Membrane Bio-Reactor.