20 lots mean 20X50=1000X8=8000 + charges is your cost.
It is same as Holding 1000 shares of a Stock called "NF_MAY4900PUT"!! If price (read premium ) goes above 8 you gain and if price goes below 8 you loose just as you would had it been an equity share!!!
The only difference between this "Share" and other Equity segment share is that this share shall be worthless post May Derivative closing if Nifty closes above 4900/-