Its easy. There are two types of cost:fixed and running
You have to find (Fixed cost/kWh) and (Running cost/kWh)
To find FC/kWh:
Find Fc first. So FC=(interest+depreciation on Capital cost)
So FC=15% of (5 crore/MW*400 MW) =2000 crore * 0.15=300crore
And total kWh(units) produced by plant in one year
=(average units produced)*(time duration)
=(peak load*load factor) *( total hours in year)
=262.8 * 10^7 kWh.
So thats it. FC/(total units produced)=(300 crore/262.8*10^7 units) Rs
and Running cost per unit is already given=50+3 paisa per unit
So total cost of unit = (FC/unit)+(RC/unit)
Do the above calculation for other types of power plant and find which one is cheap.