Here are the best ways for high level players or members to make alot of money.

1) Fletching - Start on normal trees and work your way up to Oak. Cut Oaks from now on. When you get to willows, make shortbows until you can make longbows. From then, make willow longbows until you can make Yew Shortbows. String these aswell and use high-alchemy on them to get a fair bit of cash. When you can make Yew Longbows, string them and use high-alchemy on them. You get 768gp for each Yew Longbow.
2) Combat - As high levels, you should be able to kill green dragons. So kill them (you will need an Anti-Dragonfire shield) to get a really good loot, Dragon bones (which sell for 1.5-3k each) and a green dragonhide. If you have high crafting, you can make Dragonhide armour with these or you can sell them for 1.5-2k each.
3) Theiving - Theiving is easily one of the best skills for making money. Start on people, then move up to farmers. Then move up the bakeries found in Ardougne until you can steal off warriors. Steal off warriors until you can steal off guards. Steal off guard until you can steal off Heroes. You will get about 300-500k by that time.