Killing Warped Bats

Killing warped bats can make you quite a bit of gold
simply pick up the batwing which drops about 25% of the time
all you need is to have finished the blood pact quest

Blood pact quest guide

The warped bats in the Lumbridge Catacombs drop batwing about 25% of the time. They are quite easy to kill fast, as they only have 600 life points and respawn almost instantly. Using a two-handed weapon to maximize the initial auto-attack, use basic abilities like Wrack or Piercing Shot to kill them if the first hit isn't enough. Alternatively use higher tier fast hitting weapons with momentum, such as dragon longswords/Claws, as this results in considerably quicker deaths of the bats. Pick up any batwing, and teleport back with the Lumbridge lodestone, bank, and repeat. (Also, remember to right click on loot stacks, because batwing frequently disappears under runes, Shields etc.)

Also here is a youtube video about this method 

Happy money making everyone