Hi Aabid
Since the question was asked for calculatinjg the power budget of WDM PON systems so my answer was specific to the particular systems.For other systems the formula might vary a bit but generaly for any system power budget is the difference between the transmitted power and the power level measured at the reciever end ( Reciever Sensitivity).
And for calculating the loss of each component check table 11.2 in the link i am posting.
http://ops.fhwa.dot.gov/publications/telecomm_handbook/chapter11.htm
Hope its helpful
Regards