I think even level 40's manage to kill about 75 dragon's per hour, i have bought a Bot which kills the green dragons at chaos tunnels and i manage to kill about 100-120 per hour making about 25k exp per hour at combat 97 with 93 strenght
Think of it this way -
Each Dragon is 75 HP - 750 LP
75 multiplied by the combat xp per damage is 4
300 xp per dragon
Now u go time yourself in an hour and there u go!