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From 1gp to 100k
Getting that money make started is pretty easy. The 3 methods of making 100k fast are listed below.

1) Clay Mining
This is one of the easiest methods to make 100k. Get your pickaxe and go to the southwest Varrock Mine. Mine clay until you have 1000 in the bank. Clay is instantly mined and only takes 60 minutes to mine 1000. Sell it at the Grand Exchange

2) Feather Collecting
This is one thats more complicated. Go to a chicken farm and pick up feathers dropped by chickens. Chickens are easy to kill and give 5-15 feathers each. Get 20,000 feathers and sell it at the Grand Exchange

3) Rune Essence Mining
This one requires to have the Rune Mysteries Quest Completed. Go to Aubury in Southeast Varrock and countinuously mine 1600 essence. Then sell at the Grand Exchange.

From 100k to 400k This will work within an hour. Using your first 100k, buy 1000 clay from the Grand Exchange and store it in the Bank. Now take 1000gp and nothing else. Go to the Gnomecopter Area. (North of Farmer Fred) Buy 27 JUGS OF WATER from the Hermonious Avalafrim. And store that in the bank deposit box across from it. Continue doing this until have 1000 Jugs of Water in the Bank. Now make 1000 soft clay and sell at the Grand Exchange. Then repeat this process again!