Killing Qbd is easily one of the best constant moneymaking methods in the game. Requirements for efficiently doing QBD are:60 summoning(this is the only requirement needed to acces qbd), ~90 Defense and ~90 att&str, magic or range, at least lvl 80 weapons, 85 herblore for super antifires. Also completing the quest Song from the depths dramatically reduces the damage you take while fighting her. If you can get 20 kills an hour you will make ~3,5m and it's not even hard to get under 2 minute kills which would make it over 5m an hour. To get the fastest kill times one needs tier90 weapons.