In one line
Index options trade in fixed lots, writing them requires margin the exchange computes daily, and the round-trip cost is large enough that any measured edge has to be reported net of it before it means anything.
How it works
A lot is the contract's fixed quantity: one NIFTY contract covers 65 units of the index, one BANKNIFTY contract 25. Those numbers have changed more than once, which is why every quantity this desk stores is in index points or in open-interest units rather than in rupees.
Holding an option costs the premium and nothing more. WRITING one requires margin, because the writer's obligation is open-ended: the exchange computes it daily from a risk model, adds an exposure component, and recalculates as the market moves. A combination whose legs offset each other attracts less than the sum of its parts, which is a large part of why written positions are usually held as spreads rather than as naked legs.
Costs are brokerage, exchange charges, securities transaction tax, stamp duty and GST, plus the bid-ask spread, which is a real cost even though no invoice names it. On this desk the arithmetic was established once and kept: a NIFTY futures round trip runs about 14 index points; an option round trip about 2.5 points of premium including the spread.
That figure is why this desk reports every gate result NET. A strategy that makes four points a trade and costs two and a half is a different strategy from one that makes four points, and the difference decided more than one result here.
What it is not
Margin is not a loss and it is not the cost of the trade — it is money set aside that comes back. And a low brokerage is not a low cost: on small premiums the statutory charges and the spread dominate, and a plan that survives only when costs are ignored has not survived.