That rate is called 3 mils or $0.0003/share. I don't know any customers that get that rate anymore for execution and clearing. I'm not sure what you meant by,"On a 20 USD stock this comes out to $15/million notional". If you are charged 3 mils and you trade 100M shares in a month, that will cost you $30,000 in commission.
Keep in mind that this US equity MM strategy is very competitive with many players getting directed order flow. And, non-display market data fees at co-locations like NY4 have gone up substantially 01/01/2015. See attached: