Many factors. Samsung and Sony brand itself demands some premium. Plus some of the parts used in MMX and Celkon are not of that high quality.
For eg, Say camera, it may be 8 Megapixel but what is the vendor of those cameras? The type of plastic material used to make body. quality of screws used. These are just few things. there are lot many things included.
But main thing here is premium demanded on the goodwill of the company. Sony, LG, Samsung, Apple, etc.
It is the same if, Mercedes comes out with FIAT's MJD, it would still start from 15-20 lacs as compared to FIAT itself, TATA and Maruti.