Try mining pure essence for awhile. 1,000 pure essence will get 103K.
The best ways of making money in RuneScape are by no means 'fast' to start, but that does not mean they aren't good incomes. If you have a decent combat level (around 75 or higher) and are a member, you can kill Green Dragons for their hides and bones, which will make around 30-50k per trip (the time of a trip varying according to combat level). If you have 750k spare and have completed Throne of Miscellania and preferably Royal Trouble, by Kingdom Managing with as many workers as possible involved in gathering herbs and the rest mining, you can make about 60k profit a day for no effort. If you have a Woodcutting level of 70 or above, cutting Yew Trees can make millions with dedication, and if you don't have that level, try cutting Teak trees until you have it.