Sir
I want to know about the costing of a thermal power plant of 20mw,50mw and 100mw..
And the land requirement for them...

Are you sure you mean Megawatts, and Not milliwatts?

100 milliwatts is 1/10th of a Watt

http://www.eco-business.com/news/suntech-inks-100mw-india-solar-panel-deal/

I don't know the answer, but I would think that it would really depend where you're planning to build/run a power plant and if the resources are far or nearby...

Sir I'm asking about mega watt and I suppose the plant is in haryana then what would b the estimated cost, any nearby idea..

No one here will be able to answer your question about cost of constructiing a power plant.

Try Googling :

Let me google that for you (http://www.lmgtfy.com/?q=estimated+cost+of+power+plant+construction)

Perhaps you or your firm should do due diligence and absorb the cost to perform a feasibilty study of construction cost to build a plant of the specifications required, size of output, local restricitions, etc.