@parttime, when you trade big volumes brokerage doesn't matter after a point.. I trade a lot of reliance futures, usually 10 to 15 lots at a time...
When you place 2500 to 5000 reliance at one time, if you place a market order, you could lose 2 to 3 points, but there was a time where I have lost 5 points also during the day.. So one entry and exit at market will cost me 10000 to 15000 in impact cost.. Now do the math and see if Rs 50-100/crore makes any difference even if I save a fraction of this impact cost..
If you don't trade nifty options/nifty futures today.. everything else is just messed.. One of my friends got messed the other trading hdil futures, Lost **** loads just in entry and exit for 5 lots.. I am not talking about the day when it tanked, a few days before that..