Hi Rajesh,
Johan gave you the explanation just above: this parameter is not an amount but the quantity of items for which you want to determine the price (cost or sales).
I never checked which value is B1 retrieving while filling the document's form... So just did the check (and will update my samples 
) this is the price after discount which is retrieved by the function. So my guess, is that B1 is also retrieving the price from the PriceList associated to the BP and recalculate the discount.
Regards,
Eric